The old 70s big block steel valve covers with exhaust manifolds were a pita to seal. If you got the gasket to seal on the tin, the heat from the exhaust manifolds would cook the bottom edge of the gaskets. Often the area around the bolt holes would be pulled down towards the head, and the covers would leak between the bolts. Usually, every time you replaced the gaskets, you needed to flip the covers upside down, and set the area around the bolt holes on a flat surface, and tap the covers flat with a hammer. You need to flatten all 6 holes, but usually the two bottom corners were the worst. You want to look the covers over well, after several beatings (er, I mean flattenings), they tend to crack around the cover base where it flattens out for the gasket. Those cracks will drive you nuts thinking the gaskets are leaking, when its actually the cracks in the covers themselves. If you have installed a couple sets of gaskets, you need to flatten the area around the cover bolts, and you really need to look for cracks.

With the manifolds, there is a lot of heat retained across the bottom of the covers. If you do not use the high temp gaskets, you will be replacing the gaskets at least every year. If you can add manifold gaskets, or space the manifold away from the head, even 1/8", it will extend the valve cover gasket life a bunch. At any rate, you need to be sure nothing is building up between the manifold and the cover, if air isn't passing between them, the gaskets don't stand a chance. Gene