I would go with a 4L80E there has been much development of the 4L80E - yeah it only has 3 gears and an a OD - but it is well proven in the High Performance big power arena - Rossler offeres a bunch of gear ratios and internal parts all the way to some pretty impressive complete transmissions - Reid makes an aftermarket case and there is now a 6 speed conversion now avaialble for the 4L80E