Well, if it has been line bores, then you can't take anything off the cap, as it will already be sunk into the block centerline, just as a timing chain comes in undersized models. Youll have to add some RTV or even a second strip of gasket material to the top of the rear cap to compensate for the uneven surface. Does Someome make that spiral beveled bearing that pushes the oil away from the seal or is that a smll block thing? Dont rule out the cam freezeplug or the oil galley plugs behind the trans, or even a leak that finds its way to the lowest point of the motor. Get yourself some fluorescent dye and a black light penlight and start hunting after hours or in a very dark garage.