Automatic transmissions use consume power.
I don't recall the exact difference, but in a small block powered truck that wasn't being used heavy all the time I'd go with the TH350 because it uses a lot less power than the TH400. Better mileage and more umph to the ground.
I'm building a 454 powered 1/2-ton using a TH350. I haven't driven it yet, but don't expect to have any problems with it.
