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.