Crankshafts?

383 crank will work in the 400,but a properly machined and balanced 400 crank should be able to handle 400HP.
400 rods should be okay @ 400 HP as well
If you could get the 383 stuff cheap (which most of the time people will give them to you), I'd use it. If not, you'll be fine with the cast 400 stuff (don't mix-n-match the two)
Manual cranks have the pilot hole drilled into the back of the crank. You can mount an auto trans behind a manual crank, but in order to install a manual trans that hole/recess must be drilled. It's a crap shoot if the crank is drilled if it came from an auto car, my educated guess is that you are more likely to find a 383 crank to be drilled over a 400