Sealing up valve covers

I would never use RTV and any gasket sealer together except oil pan & intake corners or you're asking for a leak.
OP never said what heads on his 318 . If Aluminum heads that's a different problem because aftermarket VC rails are machined flat where Factory Iron is a cast rail w/ no machining at all & is rough & like someone pointed out May Not Be Flat, as many of us already know.