Just another two-bits worth here. First look in my gallery at the picture of the engine in my car. You'll note it is done '60's style. No valve cover breathers. Only a fill tube in the intake manifold. Just like they were originally.
The only thing I did was substitute a pcv valve for the old road draft tube out of the back of the manifold. I dummied up the original plate with a grommet and hooked the hose up to the outlet on the back of the carb.
I did not have any leaks on the top end at all. All I ever got was an occasional drip from the
oil pan. Even that was minimal.
Now to my point.....finally. One thing many people overlook in building an FE is that most often there is a combination of parts from different engines. None of which was machined to fit together. Add a brand spanking new intake manifold which was manufactured to hit a "median" in sizing and it gets pretty tough NOT to have leaks.
The reason my engine did not leak, is that my builder carefully measured every mating surface, from cylinder deck to head surfaces and intake to cylinder heads. When he was done mating up my collection of parts, he had done a LOT of machine work. In case anyone wonders, I got to watch a fair amount of the build process. Until I watched him I actually thought I could build an engine!
Now, the point of all of this is:
Before horsing around with a bunch of gaskets and sealer, I would have a really competant FE builder (not a chebby guy) check out your top end fit. I know it may not be any fun to have to start taking things apart again, but it could very well answer your questions and get rid of the leaks at the same time.
Last note: A lot and I mean a lot of people professing to be engine builders, have no concept of what the words "close tolerances" means. Like most amatures, they figure silicone will cover up the problems. NOPE! All it does is prolong the agony.
Probably not what you wanted to hear, but I thought you should have something concrete as a basis for your investigations.
Happy hunting
Al