Does the VX engine have 6 bolt mains? or 4 bolt?
Six bolt mains.

Why are you going with a TH400?
I would imagine because the TH400 is a very robust transmission and it's still fairly short. And he probably wants to stay auto. BTW an NV4500 is like 5" shorter than the stock trans if you want to go with a robust stickshift. An SM420 or SM465 are even shorter than that, but they are only 4 speeds. All three of them bolt right up to any GM GenIII or Gen IV motors(5.3, 5.7, 6.0, 6.2, etc.)