Block Milling

I'm guessing 4 valve releifs is a flat top design. You can mill the block, but you are using the magnum heads. I'm not sure what the CCs are for them. With a std head (open chamber), and a thick head gasket, you'd be looking in the 9.5:1 range. IIRC, the Magnums are like 65cc? That sticks you at 10.5:1, using the .043 Fel Pro gasket. This is guessing the vavle pocket size at 6ccs. It sounds like you're where you want to be if they are true flat tops.