400 c.i is internally balanced as far as I know. They need a counterweighted flywheel (or flexplate) for proper engine balance. Take the crank from your 400 and find a cheap 350 4 bolt block. You won't run into issues with cylinder heads etc., will end up with 383 ci for just a little more machine shop costs. If you're replacing pistons etc., your parts bill will be less by going with a 4 inch bore. Just my opinion.