I like to use cork valve cover gaskets, all though, you only get one use out of them. Also cast alloy valve covers seal better than stamped (chromed usually) steel covers. To much flex on the edges with the steel covers. Of course, don't over tighten and put to much crush on the gasket surface.
Oil pan leak at the rear is real tough. The question is: From where is it coming? Rear main seal? Back of the intake running down? Valve cover leak at the rear and running down?
Oil has an amazing ability to travel and leave you looking in the wrong place for the leaks origin. Might not be the
oil pan or rear main seal at all...