I can understand that for sure.
How well does your glass seal?
Do you have insolation in your cab?
Does the Classic Air have AC that is directed through the heater core and then through the defrost ducts?
Can you pull fresh air up through the bottom of the kick panel into the Classic Air unit or is there no option for fresh air into the Classic Air unit at all?

On the 55 and 56, when the PS hood is up there is a vertical surface above the fender facing to the side, up to a seam on the firewall under the hood Could that be a space for a fresh air intake to the cab? Make the cutout on that vertical face, cover it with S/S screen mesh (like that screen used for gutter guards). Then add a vertical seal just forward of the screen, and above the screen, back to the welting for the hood when it is closed. Stop the hood welting just where it meets the seal for the fresh air intake. If the seal were thick enough to require some compression when the hood is closed it would seal off the fresh air intake from the engine compartment. But fresh air can enter under the bottom edge of the hood just above the RH fender. So the screen is covered from outside elements and separated from the engine compartment heat. Inside under the dash area form a duct that rises from the bottom edge of the fresh air intake so that any moisture that could get in would have to travel uphill to get into the ducting, so that should minimize any chance for water/snow to get drawn into the cab.

This would be similar to the fresh air intakes for the cab on my 2001 Dakota. When the hood is closed a seal forward of the intake area is compressed by the underside of the hood, thus sealing the fresh air intake from the engine compartment. But the trailing end of the hood extends past and over the fresh air intake to keep water, snow, ice, etc from flooding into the cab fresh air intake. Whatever the size is of the ducting coming out of the cab blower, make the intake tubing about 1.5 times that cross section to make sure the blower is not working harder to pull in fresh air. The fresh air intake duct would then go to the blower housing suction side where the fresh air/In-cab air damper would be.

Poorboy, on your installation where you show the cutout under the hood for the fresh air intake, the area I refer to is further to the left (looking back to the windshield), lower below the firewall flange and vertical, versus where your cutout is higher and above the firewall flange under the hood.

I responded by PM, but for anyone that might be reading this, the 48-56 hoods close on top of the forward 1/2 of the cowl. The hood sides are support by a center piece, and both sides are held closed by latches that pull the hood down against the cowl and the bracket at the radiator support. The cowl part behind the hood is exposed to everything outside. The cowl part under the hood is exposed to everything going on under the hood, and the entire cowl has to support the rear end of the hood sides. The original seal for the rear of the hood was a welting that as nailed through the cowl. Most replacement seals are rubber, but the welting is available for restoration purposes. The front part of the cowl seals the engine compartment from the passenger compartment.

You can remove that under the hood part of the cowl forward of the step for the hood seal, but you will have to figure out a way to seal the engine compartment from the passenger compartment, and you have to provide enough bracing to support the rear of both hood 1/2s and the center hood hinge piece. You can pull the fresh air for heat/defrost from under the hood, but you will want to keep that opening to a minimum because you will pull under hood odors, under hood heat in the summer, and under hood cold air (until that air warms up) in the winter. You also create a possible place for an under hood fire to enter the passenger compartment. A small hole (about 4" in diameter) will provide plenty of fresh air for a heat/defroster, and will reduce the chance of an under hood fire getting into the passenger compartment. A hole that size can also be filtered to reduce unwanted odors, and a closable door can stop unwanted heat in the summer.

Most modern outside vents pull outside air through louvers in the hood, or gaps between the hood and the windshield. There is a box under the fresh air intake with water drains and a protected from direct rain area above the expected water line where the fresh air enters the cab. The heater boxes usually have a drain at the bottom to remove any water that has gotten into the box. Modern heat/ac units are designed to pull required fresh air (or recycled air from inside the cab) into the system with as little added moisture as possible. Then they add a way for any moisture that has entered the system to get out before the fan can push that moisture against the glass. Ac systems both cool and dry the air. Modern heat/ac systems funnel the cool dry air through the defroster system to first dry the air, then pass that air through the heater to warm the air. The dry warm air is forced against the glass as fast as possible. The warmer, the dryer, and the faster the air moves, the faster it clears the glass. A system without fresh air and without ac can clear the glass, but it needs to get rid of the excess moisture to be more efficient.

If your glass is slow to clear, its possible there is too much moisture in your cab. Poor glass seals and poor door seals are a good place to start to reduce the moisture in your cab. If water is coming it, you have to figure out a way to at least slow it down as much as possible. Total seal is the best option, but these old Dodges had lots of gaps around the glass and doors.

Condensation is the next biggest culprit. If you don't have some form of insolation stuck to the inside of the cab sheet metal, you are probably adding moisture through condensation forming on the inside of the metal that is warming inside but cold outside. Your defroster fan is pushing that moisture directly against the glass.

Wet carpets? Brush as much of that snow off as you can before you get into the truck, and fix the holes in your floors.

the air coming out of your defroster has to be as warm as possible, and it has to be moving as fast as possible. Most of the old heaters have fans with blades moving the air. Modern fans have squirrel cages moving air. Squirrel cages move the air about 2x faster then fan blades will. Are the duct work and vents designed to move as much air as fast as possible? Smooth surface walls and easy curves move more air then rough inside surfaces and 90 degree square bends.

Lastly, is you glass clean? Dirty, smoke, or antifreeze coverd glass takes much longer to clear then clean glass does.

.

Glass seals fine. Moving the fire wall didn’t impact the windows.
Yes I have insolation. I replaced the original fire wall and floor with a firewall and floor from a Dakota. I used the Dakota insolation.
Yes the air is directed through the heater core then to the defrost ducts.
There is not enough room to pull air in through the kick panel into the classic. So that wasn’t an option.

My 54 Dodge originally had a hole (about 3" diameter) in the firewall near the heater for fresh air. Under the hood was a short tube with a butterfly valve that was at one time cable operated. I suspect originally it had an expanding paper/wire tube that connected it to the grill area near the radiator. Even a hole connected to the Classic Air unit with duct work through the firewall and going towards the fender would help a lot. Any fresh air would be better then none.

I just saw a video of a 55 Ford F1 build onto a newer chassis. Just realized what those louvers are for on the PS of the cowl just aft of the hoodline, most likely for the cabin fresh air intake. The louvers are only on the PS of the cab. Always wondered why the old Fords had those louvers on one side of the cab. That would be cool on the 56 Dodge but a louver section would have to be welded into a cutout section of the cab side cowl to make it work.

Mid 50s Chevy and GMC trucks had a small door on the side of the cowl that opened & closed from inside the truck for the fresh air. That part of your Dodge truck is covered by the back part of the front fender, and any opening would open between the cab and the back side of the fender. On the original Dodge trucks, that area was also exposed to anything the front tire could kick up and send back into the area. With modern inner fenders that could protect the possible fresh air door location from the tire kicked up stuff, you would be pulling the air from under the bottom of the fender or between the engine compartment and the inner fender. That probably wouldn't be a bad place to pull the air from during the winter, but it could be pretty hot air during the summer. Rigging up a door to open and close in that tight space could be an issue (it could open inward where there may be more room, or outward). Inside of the cab, that location would be the passenger side kick panel between the dash and the floor. That air would then have to be ducted into the HVAC box. On my 49, I just moved that fresh air intake to the top of the cowl, which made the duct work easy. I could have probably taken the time to create a door that would open and close, but the cover for the summer was quick and easy, but the cowl fresh air vent on my truck still functions for the summer use. My AC is not functional at this point yet, but may be in the future.

I might also add that I have my EFI computer mounted in that passenger side kick panel area on my truck. Locations you can place the computer are pretty limited on the old cabs, those wires only reach so far and since most measure resistance, making the wires longer probably isn't a great idea.

My 56 C3 today was flat bedded from the seller's driveway to my FIL's pole building, now resting quietly, safe and dry amoug his collection of Jodn Deere, Oliver, Massey Harris and Ford tractors.

Russ and Gene, your cool Dodge classics on Dakota chassis's are my inspirations to do my 1st chassis swap. It will be a long and challenging build, and I hope to have a truck close to what you've both built. You both should be very proud and satisfied with your efforts. Maybe someday I'll park my turquoise 56 next to one of your trucks.