If the AC unit you pick up still contains the old refrigerant inside, expect to pay dearly to have it retrofitted to the new R134a. I have been quoted between 5-600 bucks for this job. As well, installing another AC unit should be vacuumed and cleaned anyway, new dryer would be wise as well. Then you have to refill them.
Auto AC is becoming a very high priced item for updating to new stuff.
As for fitting...It depends on the mounting brackets from 63b to 4g64. There will probably be some challenges I imagine.