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.