There is another choice.
Take the Fentons to your favorite machine shop and have them grind the entire amount off of the mounting surface that bolts up to the head. Both headers , the entire surface the entire amount you need to grind off. In other words , you resurface the ports of the headers.
With this procedure, two things happen; the tabs are then thinner as you need them and the headers clear the Offenhauser intake because both headers are about 1/4" closer to the block.
