LS3 6 speed vs LT1 8 speed

Trying to decide which I should go with for a conversion. The target vehicle is a 2003 Dakota ext cab 2WD, currently weighs in at 4000lbs with an iron 3.9L V6 and 4 speed auto. I have to go with an auto due to a bum knee, shifting's getting to where it means limping these days. The eventual goal either way will be in the 700HP range, and the suspension, brakes and rear are being built to handle the additional power, and I know what all has to be converted over. The truck will run a 29 inch tall tire, and will have either 3.42, 3.55 or 3.73 with a positrack in a Ford 9 inch connected via an offset 3 link and coilovers. From what I can tell, the 8 speed with the 3.55 will have just about the same shift speeds at each gear as the 6 speed with 3.73, and the 6 speed is like the 8 speed except that it's missing the 8 speed's gears 2 and 5.

My question is, which engine/trans would you go with and why? If I could put an 8L90E behind an LS3 and make it work I wouldn't be asking this, but since the LS3 only works with the 6L90E and the 8L90E only works with the LT series, I'm stuck. Money isn't an issue for this because I can get either as a complete takeout with the wiring harness and other parts for about the same money from a boneyard.

Or, if anyone knows if there is a way to make an LS3 work with an 8L90E, that would be great too. In fact, that would be the perfect solution.