If you have the opportunity place some heat reflective insulation between the roof and headliner it makes a huge difference in our dark green wagon. The original factory units were huge under dash units that were installed at the dealer and can be found in many other US cars from the 60's. Replacing the compressor with a modern one will be more effecient than the lawn mower size original. So if your looking for a cheaper option seek out an old one in a parts yard, replace the fan blower motor and use the rest of the modern parts, however it still won't compare to a modern Vintage Air unit.
I have an original under the dash unit that I can measure for you if you want to compare...still has the install sticker from june 1966 in phoenix!

Good luck.
-Erik