A few questions about the crank that's in there. Your running a 410 so i'm guessing a 390 block with 3.98 428 crank? Double the
oil hatch marks on the rear of the crank. These are the //////// marks on the rear of the crank and keep the
oil from running outside the seal and south from there. Not sure if your engine was running fine before and just started to leak or a rebuild. If it's a rebuild check the crank as they might have been machined or smoothed. Also check for anything that could catch and tear the seal. Either one of these would help
oil out of the block as well. I also second the plugs by the rear cam plug. Some early blocks used the press in plugs as well and should be replaced by threaded. Also good teflon sealer on the threads. This will simulate a rear main leak and is something to think about. The bad part being the engine needs to be raised at least if not taken out of the car to replace them. Are you using the rope or neoprene lip seal? G.