It's been said before that Ford didn't make gaskets.
Chris Allison recommends Elring still as far I know, but you must clean the mating surfaces with great attention.
I used an Elring one when I had the Thunder, spent ages cleaning the surfaces and it didn't leak.
I replaced my gasket last year and I needed several parts for other work, so as a Ford employee I got everything from a Ford dealer. The parts agent confirmed the ‘Ford’ gasket was not made by Ford (to be honest Ford outsource the manufacture of nearly all parts except main components for transmissions, engines and body) and recommended a 3rd party gasket. However it was a poor fit and I ended up re-using the old one but using Loctite MR 5922 flange sealant to secure the gasket into the rocker cover so it would not pop out during assembly, and then an automotive silicone sealant on the mating face of the gasket just before assembly. As others have stated, make sure mating surfaces are really clean.