Welcome to this forum! I've moved your thread into the stock 91-94 section. Stop leak chemicals are like a temporary patch since they cause old seals to swell. Replace the gasket with a new rubber gasket such as the Permatex style. They don't use any sort of adhesive to make the bond. Unbolt the valve cover, scrape off the old gasket, then install a new gasket, and don't over tighten the bolts.