Only do the 3.91's if you are cammed or have a triple disk aftermarket converter. Otherwise do like others said and get the 3.70's. I dont consider wheel spin from 3.91 gears to be the biggest issue as you could always use bigger and stickier tires. Its the fact that it forces a shift into 5th too soon if you dont have a cam with a higher redline or triple disk converter that can be locked up at WOT.
BLK/BLK 1SS/RS Ordered 11-01-2009 Took delivery 12-22-2009. SOLD Feb 2015. Waiting on 6th gen LT1 SS with 8L90E.