Had a similar problem with a Ford Y-block last year. After much frustration I finally found that someone in the past had ported the head and got a bit carried away and ground the intake runner paper thin. Finally it cracked and was allowing oil to be sucked into the head, causing that cylinder to smoke and fouling the plug. It wasn't in a spot that could be welded, and the owner didn't want to pay for a new head, so some careful work with some epoxy sealed the crack. A year later it's still running strong.