DON'T OVER TIGHTEN THE ROCKER COVER!
Sorry to shout but its important. These old things have 4 screws holding the rocker cover on right? They don't need to be extremely tight, just enough to seal on the gasket.
Otherwise the cover warps and it will never seal again. This had happened on my escort and made me mad because I couldn't get rid of the oil leak.
not that you will ever get rid of all the leaks on an old ford, its how you know they are still running right