.050-.060" would certainly get you a few tenths of a point over stock. The best you could have stock is around 8.0-8.2. Starting at 8.0, and cutting .060" and using the .028" thick gasket will get you up to 8.2-8.3. Beyond, that, you need a piston change or deck work. Sealed Power 526 is the best price for a low cost cast unit. (But it does not sound like a piston change is in the plan for your 318; very understandable.)
How you would mill the intake and then go back and use it on the 340 is not making sense. If you mill off of the head surfaces instead, you take .0095" off of the intake faces of the heads for each
.010" taken off of the head's surface to the block. (This is per the Direct Connection engine mods book). Milling the intake is a different number.
