Its definately not coming out from up top. It either has to be the rear seal , retainer, or oil pan gaskets, Unless it is coming out of the cam plug or a oil galley plug. I might pull the tranny out this weekend and see if I can see anything on the back of the block. I have 3" exhaust with Dynomax Bullets and turns out in front of the tires so I would think it would pull enough vacuum. Do those evac kits seem to help?