400 Vs. 440 into an A body

383's & 400's are 1/2" shorter than 440's and use the same mounts. so really not much difference to speak about as far as putting them into the A Body. it takes the same amount of work & modifications

if you find a nice 440 crank and put it into a 400 block with the correct pistons you have a 451 stroker.
KB 215 with 0X0 at the end of the part number... the X is a 1 2 3 4 etc depending if your block is 10 20 30 40 over etc
http://www.houstonmopars.org/stroker.html

if you decide not to do the stroker you use KB 240 pistons not the KB 215's and you have a 400 at 10:1

either way will have plenty of power, you will just pay more for the machining to do the stroker, since they have to cut the crank, and make sure the rotating assembly has clearance to spin, they may have to trim sections of the block.

the heads / intake on the stock 400 are not as good as some of the 440 heads / intakes