Your 68 heads will either be 54 or 58 cc's depending on the casting#'s and the production tolerances. The E7's run about 63-65 cc's. According to Ford, you can mill them up to .040 without alignment problems. I milled a set of Canfields, .060, the block was decked .005 and I used a Felpro race head gasket and had to mill my intake. How much you may need to mill the intake will vary. It's best to ( as well in pushrods also) to do the final assembly, then set the intake down without gaskets, then using a feeler gauge, check the fit before milling the intake. When I bolted the milled Canfields in place, I tried the Ford A321 intake I had on before the head swap and the Edelbrock Vic Jr. I ended up using and the fit was different between the two. What you need to check is how the manifold to head faces line up and the distance under the intake to block gasket rail.