The other thing to remember (having just gone through this on my 261 rebuild with Jerry) is to make sure the mating surface on your valve cover is flat (where it contacts the gasket). I converted to use a 216 valve cover with the acorn nuts, so we wanted to be sure the valve cover was flat. Jerry had showed me how to do it on the oil pan, so I worked the flange on the valve cover the same way, put the gasket (cork, BTW) on dry, placed the valve cover on and just snugged the acorns and no leaks yet. We've run the engine on the test stand now for about an hour, so not a conclusive test, but I think if it was going to leak, it would have done so by now. I used the 261 gasket that was in the gasket set, but I used the box cutter out of the rear main seal kit to trim the 4 bumps off (where it surrounds the 4 screws on the 261/235 valve cover.
Just offering my 2 cents -- if you still have leaks, you might check your valve cover for flatness.
I hope this was helpful
