In my 73 I am using the factory original AC/heater box. Firewall forward, I got a kit from Bouchilan Performance. Sanden compressor and mounting brackets, filter dryer, freeze switch, condensor coil, and all fittings and hoses. It is an r134 system and blows ice cold in 110° heat.
They key to the entire system is in the heater control valve. New ones suck and do not completey stop the hot water from circulating in the heater core. The AC will never blow cold like that. Randy at Bouchillan told me to disconect the heater hoses and see what happens. It blew ice cold after that.
My 72 Sat being restored right now is factory AC car with a busted heater box. I would like to use the smaller aftermarket under dash system in that car. I too dont know which ones work the best, I hope the answer comes here.