Stock valve covers are made for stock heads....... which have the bolt holes/studs not perpendicular to the v/c rail, using gaskets that are fairly compliant so they seal against irregular surfaces.

My guess is those v/c’s would work just fine on a set of 906’s with some cork gaskets.

68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads