The biggest culprit is the fact that real Freon R12 is only way to go and it is readily available you have to ask for it and you have to have someone with a mechanics license to get oreilly's has it,I have done 3 gbodies,Also was schooled in auto a/c,when you convert any older system to 134a it will not cool as good or nearly as fast in stop & go driving in hot weather in a older system like in a gbody they just weren't very efficient systems to begin with,and that stuff called freeze 12 is not real R12 it is junk.
always go with a new compressor the long style one works better for sure,the short radial ones don't last very long, I am in the process of doing my 80 G/A right now and converting the compressor to a sanden 510 which is a bigger heavy duty one more like the ones of today and the stock blower leaves alot to be desired I always replace it with a 2 piece one with the metal fan it pushes more air (O'reilly's)and clean the air box out really good and use strip caulk for the seal on the blower to the box, if the a/c has not been used in a long time check all hoses I would replace the main hose and all orings and use R12 oil ,rub a little on the oring before you install,put about 4 ounces in the compressor 2 or 3 ounces in the new dryer,it doesn't hurt to use a couple ounces more than what is called for, the compressor will last longer, if the old compressor went out it is good idea run a/c solvent thru each component that use reuse condensor, hoses etc. pour a liitle in and hold a rag at one end and blow with air hose, after assembly pressure check before charging.
good luck