383 or 400 ?

The only way a 440 piston can be used in a 400 is if you put the same stroke crank in it too. Hence the 451, 470, 496, 505 deals that use the same pistons rods and strokes.
If it was me I'd run the 383 and either keep or sell the 400 on. The 400s worth something - the 383 isn't.