This is an age-old problem that has plagued SBC valve covers since the V-8 first appeared in 1955. There are only 4 small 1/4" bolts attaching the cover. These 4 widely spaced bolts cannot evenly spread the clamping force over the entire area of the gasket surface. The gasket surface is not machined. Rather it is an "as cast" rough surface. Couple these factors with the contour of some of the various exhaust manifolds over the years that are routed in close proximity of the gasket, and expose the gasket to high temperatures, you have a perfect leak prone design..
Over the years, I very seldom used RTV based adhesives in conjunction with the latest gasket offered. I adhered the gasket to the valve cover with 3M weather strip adhesive, and attached the valve cover/gasket assembly using slightly longer bolts and the
"re-enforcement tabs" in this link.
https://www.summitracing.com/parts/sdm-pce312-1001