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.