Probably doesn't need to be said, but before you go bolting the cover on or tacking the gasket to it, make sure the bolt holes around the cover have been put back to flat. Sometimes when the bolts were tightened too much the cover isn't flat anymore and that causes a leak... A straight edge around the cover will help identify the problem areas, if any.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi