Think I'll go for the 4x4 manifold.
My plan is also to stop around 400bhp, this way I can go with my current gearbox and diff (as long as it holds). Try to let it role before pushing the pedal to the metal, to save the diff.

With my 2wd (non small turbo) engine I think I don't need the thingy under the main bearings.

Quite some impressive graphs you have there by the way m8