Instead of vintage air, have you looked into the rear AC unit from a large (3row) SUV? Tahoe / Suburban / Escalade have the cool ones, but some of the large Vans have high BTU ones. The plus side, is that some of them have heaters built in and can be found in recycle yards for a fraction of the cost of a VA system. We've had good luck with those inside of vehicles with small dashes or behind the seats of regular cab minitrucks. Weld up some tiny (flat) pathways and run the "feeds" to the dash, under the seat(s). The trunk on that thing has some nice areas that you might be able to hide the actual unit, if under dash / hood space is at too much of a premium.
Looking forward to this project, restored a 66 and 64 1/2 a decade ago.
