I added factory a/c to my 69 cuda from a parted out car. It was a lot of work and probably not a job for a novice. I would imagine it would be similar for an early A. It’s nice now that it is done, but it would have been 10x easier to go aftermarket, but about twice the cost or more.
Don’t believe all the stuff about R134 being inferior. I run it in my 69 with no modifications. It gets so cold I have to turn it down. I have been doing it for years in multiple cars. No changing o rings, oil, hoses or any of that crap. All that is is other people making money on you. I think the problem most have is they don’t pump the system down right and don’t charge it right. The obvious out is to blame the r134. I’m not saying that the R134 works as well as the R12, just that it works well enough you won’t notice if you have charged it properly.