Possible sources are the manifold-block seal, particularly the corners at the head, upper left bellhousing mount hole, cam plug in back of block, and the most probable - rear main seal. The main seal will usually leak more inside the bellhousing then drip out where it bolts to the plate, leaves the flywheel clean. If
oil is only on the front of the plate, the manifold joint or the loop gasket in the pan around the main cap move to the top of the list.