I baught mine used, pre-modded with a ZZP 2.85" Pully,
What's the stock Tensioner pully size?
How can I tell if I need a stock belt, or a
Stage 2 belt?

 

Well first off your about 4 pulley sizes away from the stock press on pulley that came on the blower. With your setup I assume it allready has the gm stg 2 belt. Stock belt wont work unless you run the zzp 3inch tensioner I think. Stock pulley tensioner is 2.5. Regardless if you need a new belt get a gm stg 2 belt, if your haveing a belt slip issues with the new belt buy the $10.00 zzp tensioner pulley and that will fix that. I havent had a issue with the belt on my 2.7 so take it for what its worth..