It sounds as if you already have the block, bearing spacers, crank, rods, and pistons. If the pistons are for the 400 rod length, the block is a 400, then the only unknown is if the piston skirts will interfere with the crank counterweights. This should be easy enough to assemble one piston/rod and install it on the crank and rotate it to check (don't even need the rings to check).