The best valve cover gaskets I have used are a little pricey but 0 leaks on 3 sets on 3 engines.. Moroso Permalign light skim of ultra grey on each side no more leaks.
Moroso Perm-Align Valve Cover Gaskets 93050 318 version.
B/RB 361--- 440
Moroso Perm-Align Valve Cover Gaskets 93055