I'm not sure on the magnum block, but I built a 408 in an la with a .030 version. with the 1.41 comp. height and had to deck the block approx. .050 to get near 0 deck. Which if your looking for 9/1 compression you won't need to deck it. I want to say the rings were 5/64's also. I put them in at .0045 piston to wall.