Will a 350 torque converter fit a 400 turbo transmission?

If you're talking about a TH400 transmission, yes... the TH350 torque converter is one inch smaller in diameter than the TH400 torque converter (12" vs 13"), and you'll have to make sure the holes on the flexplate line up (or drill them for both patterns). You'll get a bit more stall out of the 350 converter. If you're talking about the 700R4, the answer is going to be no.... that uses a lockup converter, which will not interchange with the TH converter.