I first used a Gatorback belt (Stock size with 3.5” idler) when I first put on my 3.4 but that got chewed up in 6-7months and had stretched out a lot. Then I used a Napa Gates belt and that held up pretty good had almost no belt slip on my 1-2 shift with it but my sc pulley built rubber in the grooves and it made more noise then the Gatorback. The Gates Belt I had held up great till it fought with an idler pulley and they both got knocked out. So now I'm back to the Gatorback ( 715 ) and happy... Sure it wears faster but its quiet and I don't have all that rubber on my pulley, changing a belt twice a year is ok for me.
1998 SSEi The Executive Hauler
Built Trans w/ 3.29s, P&P Jenny 5, S1X cam, Silver TOGs, Northstar TB & LQ4 MAF, FWI, Dyno tuned, #60 injectors,Oil Pressure Kit, 2.9-3.4MPS, MagnaFlow exhaust, 3" HF cat, MSD Coils & 8.5mm Super Conductors, AeroForce, only Sunoco 94, 100