I'm obviously coming in late on this thread and haven't read all of the posts but read the one about using RTV on the valve cover. Old vintage Triumph guys like me use Yamabond or Hondabond #4 on it on all mating surfaces. I use it on the cases, base gaskets, rocker box and all side covers. I even used it to seal leaks on my old Shovelhead and my Evo primary case cover. It works great. I learned about it from an old Triumph mechanic friend years ago.
It is a grey paste that does not spread when the mating surfaces are pressed together. Spread it on both the gasket and the mating surface, let it set up and then join the pieces. To split the surfaces, squirt on some carb cleaner and then tap it with a rubber mallet.