I use the Fel Pro rubber gaskets too with no leaks on my 250.
I've also found that it is worth the time to check and make sure the bolt holes on the covers aren't depressed or they will bottom on their bosses before the gasket makes up snug. When I have a block stripped down to go to the machine shop I also spend some time with a large flat file and lightly go over the side cover and valve cover surfaces to knock down any high spots. I've even found side cover bolt bosses that protrude out farther than the gasket rail. Check with a straight edge and file them down to just below flush. You'd be surprised how uneven they are. Of course you don't want to be doing this on a completed engine.
With good surfaces and the Fel Pro gaskets and proper torquing of the bolts I don't have any leaks.
