Take a 4.09 bore 392 and bore it to 4.125 and open the end of the rod up to chevy size and you open up a plethora of 400 SBC piston choices. A true flat top with no valve reliefs would make good compression and you would be over 400 CID. Or better yet turn your crank down to 2.0 chevy journals, buy cheap chevy rods and a flat top 400 SBC piston and get even bigger by offseting the 3.75 stroke to about 3.87 with the 4.125 bore to come out around 414 CID. If you really want to do it in the 5.7 block I think you could find an LS piston and do all the same "chevyizing"end up around 380 depending on exact bore you settle at.