Best sealing valve cover gaskets

I hope you got your leaks contained! Just ran across this thread, and figured I'd let others know what I found. I installed my 340 back in '89 and always had valve cover leaks. I finally found the problem - there is/was a bulge on the lower center valve cover bolt hole boss! It was at least 1/16" tall. The engine is apart now, so I ground it down flat, and when I put a straight-edge to it, I found that the area to the left of that bolt hole was also taller than the rest of the rail! I ended up hand-filing it down. It was maybe 1/32" or less. And I found that on both heads! Attached is a pic...it's hard to see, but the bump is just 'above' the bolt hole and slightly off-center.

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