[cooperhw]

The reason they ask about the stock (to the LS motor) compressor or aftermarket is because the stock compressor often times has a "variable output compressor" (probably not correct word) as opposed to the aftermarket unit will either be on or off with no in-between. I would recommend using the aftermarket unit. I tried the new compressor (came with the accessory drive kit from GM) on my last build and finally took it off and replaced it. aftermarket unit worked well.
Call Vintage air. The Tech guy is happy to talk to you and explain it better.