Hi Rod, greetings from McCook!

I have been studying this situation myself and the limiting factor that I have found is the evaporator and the fan/ducting to route the air. A cheap retrofit (at least for me) would be to use an evaporator from a 40-50-60 series John Deere tractor. This evaporater is about 5" square and about 30" long. I would mount the evaporater and fans under the seat and make some ducts in the floor board to route the cool air up to the dash. The 70 series John Deere tractors use a setup like this and it works good.

The hardest thing about mounting the evaporator under the dash is the lack of room. If you get an evaporator small enough to fit under the dash, will it be big enough to keep you cool? I need to go somewhere and look at a vintage air setup installed--I have never seen one in person.

I had part of the A/C apart on an 88 GMC pickup one time and that set-up might work under the dash. It is a lot smaller that the A/C units on the 70's-80-s Chevy pickups.

Remember 9-11-01--God Bless the USA
JUSTICE, not REVENGE, will prevail

1951 Chevy 1/2-ton Pickup truck