http://www.rodandcustommagazine.com/techarticles/0910rc_retrofit_vintage_ac_systems/photo_11.html

The Mark IV underdash unit has been around for a long time and has been brought back by Vintage Air. This one would be good if you had a good heater and defrost system and just needed to add A/C. They would put these in full size cars in the 60's, so it should pretty much freeze you out of a pickup cab. It is smaller than the Cool Pack that would have been available on your '63. Pricing should be pretty good as well.