A good, sonic tested to make sure the cylinder walls are good, 400 block has a lot thicker main webs than any stock 440 block, Mopar made them that way for a reason The 440 blocks have weak main webs and will crack once you get above 600 and detonate the motor even a very small amount, been there done that Add a set of after market steel or aluminum main caps and go crazy, I have My last pump gas 400 stroker motor, 518 C.I.,made 727 HP on Oregon pump swill, it had a bunch of good parts in it also You can do the same thing, use a 4.250 or 4.300 stroke crank made for a low deck with BB Chevy rod journal sizes and a decent set of long forged steel H beam BB Chevy type rods, (6.700 works very well ) rods and shoot for under 11.0 to 1 on the compression ratio with aluminum heads, I shoot for 9.5 to 1 or under on all my pump gas iron headed motors. Lots of choices out there, make the correct ones and have fun