I would still guess valve covers. Before putting the aluminum ones on my Fairlane i fought that with the chrome valve covers. Was damned sure it was the rear main seal... but that didn't fix it. Took the intake off and resealed that (used cork ends this time)... nope, after having the bellhousing off a second time (and headers to get it off) i noticed a very slight wet trail on the back of the head. The valve covers leaked and dripped off the middle of the bellhousing. Sure felt dry from above when i'd checked.