... valve cover oil leak that must have been going on for a long time ... I ordered an aluminum valve cover to replace the plastic one, but am thinking to try double cork gasket before the switch. ... Has anyone on this forum found the double gasket approach to work with these plastic valve covers?
258 leaking plastic valve covers are so much of an issue that manufacturers were (still are?) making double thick cork gaskets, so you don't have to use two regular thickness gaskets together. RTV on upper & lower sides of gasket, thick or thin depending on who you talked to. Worked on mine - for a while. I gave up and made my own steel cover.
Will it work for yours? Depends on how badly warped your cover, and if there is any damage to the groves along the sealing edge (famous for bits of those to break off) or to the ledge that the discs to clamp down on. If it's not too far gone now, it's just going to be getting heated, more brittle and further degrade.
A more extreme solution, is to get a 4.0 head with its aluminum valve cover. Can use the stock 258 intake manifold, but needs a different exhaust manifold, typically 4.0 headers. I think this is covered in the Eaglepedia under upgrades. Then with a 4.0 head, the engine can benefit from a different cam. Slippery slope on upgrades...