Damn header leaks! wtd?

I double up Fel-Pro header gaskets and use the high temp copper RTV. After 20k miles I am just beginning to have a little leak.

When I first put in the 360 I used the gaskets that came with the headers (Hedman) and they leaked like a sieve. I tried the manifold gaskets that came with the rebuild gasket set and they were worse, the gasket is open between the two center ports and gives a straight shot leak path. I got a set of Mr. Gasket Ultra Seals, they were better but still leaked. Tried copper gaskets but the raised bead does not line up with the raised surface on the header flange and they leaked. Had always had good luck with Fel-Pro and decided to order them and also decided to double them up. It was also recomended to me to use a sealant. That worked and has been good since 05 when I put them in.