For a restorer who wants AC the factory parts are necessary. Otherwise an aftermarket system will be better. So you may find mixed opinions on the factory components. The good thing is they used better quality build and materials on those things -- I'm using the original in car components (housing, vents, expansion valve, evaporator) on a 63 Rambler with a modern Sanden compressor and condensor. You have to have hoses made (new components use o-rings, old ones beveled seats), but everything works even with R-134a.
As pmeisel said, AC was rather rare in those days for most cars. 20-25% would be my guess. Depends a LOT on what part of the country though! There will be a higher percentage of southwest cars with AC than more northerly (cooler) locations, and more high end cars (like the top of the line Bel Air). you have to remember, AC was like a $600 option back then, and a mid range Chevy would have only set you back around $3,000 -- $600 is another 20% of the cost of the car w/o AC!!