Just re-reading this and I don't think I see anyone mentioning this. Some smallblocks have a vent/filler on top. it is vital this is free. It was Sign 216 that first brought it to my attention 150 years ago, and mine was blocked literally from new. A good solvent degreaser should do the job. Although mine hadn't started leaking, now, after a good long 500km ride I now find a faint oil mist around it. If blocked it can pressurize the rear end and encourage oil past the seal