Back when the cars were new, or were "used cars", adding add-on a/c usually included upgrades in two areas, sometimes. One was the alternator (or instructions for increasing the idle speed a bit) and the radiator (making sure it was clean inside and such. Usually, Vintage Air and others have some sort of check list of things to upgrade or make sure they are working correctly so that their systems will work as designed. Might check on their installation instructions, which might be in their website?
Enjoy!
CBODY67