The GMPP Stage kits had nothing to do with the clutch. They were a tune, injectors, pulley, belt, and the multipass cooling plate (if I recall correct on the last one.. stage 3 may have had a tad more).
You can put whatever clutch you want to put in your car at any time.. providing it's made for the car... Also, make sure you check out Crate Engine Depot instead of AAS..they won't steer you wrong.
multipass cooling plate only came on the stage 3 IIRC. I know the stage 2 does not have it. (because I have the stage 2, but the dual pass I bought later and haven't put on yet!)
stage 3 also was an a/c delete and better tune
stage 2 was injectors, belt, pulley, tune
stage 1 - I think was just the injectors and tune.
Easiet way to check on whether it has stage or any kind of upgrade is the pulley. Almost all upgrades involve changing the stock pulley. If it's not stock, there is some kind of upgrade. If it's stock pulley then you can be pretty sure it is stock engine.
Then check who makes the pulley system and go from there. GM system is easiest to spot.