I would look for something a little more robust if you are planning for 400 ft-lbs. I guess if you are on street tires and not horsing around too much that trans and rear would probably be okay, but their life will be very short if you ever decide to use all 400 ft-lbs of that torque. For now, I would run the gears that came in the S-10 rear you're using. You haven't picked a camshaft yet or a tire size, so it's hard to pick something this early in the game. Gears can be changed at any time.

"The first rule of overkill: You can never have too much overkill."
"Overkill is underrated."