Once while I was working on the rockers, I had a leak with my MP Covers and a 440. What I did was use a VERY THIN coating of high-temp ATV on the covers to hold the gasket in place. Once it was dried, I fitted it onto the heads and lightly tightened it.

Ever since then can remove the covers and replace them and have experienced no leaks.

I would get a new set of rubber gaskets and try that.