I have been able to double up the older cork gaskets by using an appropriate adhesive and this has given me enough clearance on my 468" Gen IV in my 69 Impala. This works with the older style valve covers that have a flat surface where they attach to the heads. The stock type Gen V and VI covers that come with those engines have a rubber string-like gasket that won't work unless you machine the covers to permit use of the old style.
I also have seen the roller rocker poly locks machined to permit clearance but that may be a little bit of an expense that may not be worth the trouble.
Good Luck