I used Percy's and other than one BURNING OUT. And them not responding to my emails concerning it. They seal great, just don't expect any after the sale support.
I went with a set of Mr Gasket Ultraseals on my old 5.0L and my current 4.6L. I've never had or heard of anyone having problems with these gaskets. They were cheaper than the stock ones, to boot.
Look in the classifieds for them, most people who buy head swap kits don't use the header gaskets that come with them. I have like 3 sets laying around somewhere