What I would do is get the heads he wants, cc them, put them on the block to see if they and the intake fit, and determine how much milling you can do to fix the compression if you get the wrong piston in a kit. My edlbrocks are 58cc exact, others have reported some as high as 62. What little I have messed with the magnum the 1.46 compression height piston will be .005ish in the hole, Icon makes a magnum head piston at 1.465 ch, with valve reliefs, but compression is 13.5. My pistons are 1.46 ch, 28cc dish, 9.84 to 1, after a .010-.015 head mill it will be 10-1. My block is almost mirror deck finish, but the heads are rough, not confident the cometic gaskets will seal, only reason to mill them. Nobody around here can do much less than .010. Your biggest issue is getting the compression right, so pick a head and piston first and mock the block up, that way you are not milling the block, unless it is crooked, doubtful on a magnum, then the pistons, then the heads, that wastes$$$. I never buy anything internal till I put the heads and intake on and cc the heads anymore, I have learned. The only other issue is h-beam mopar rods need an eyebrow bore notch, I am told if you order the kit with chevy journals it does not. Plenty of pistons out there just do your math.