I've been down this road - after cleaning and repainting the bilge, and replacing seals and checking every fitting, the first time out there was the hypoid gear oil again!
Lots of light and a good mirror revealed a tiny hole in the transmission case seeping an amazing stream of oil. The challenge was that it was on the bottom, and not big enough for push a sewing needle through. It turns out that castings in the 1960s weren't quite up to modern standards, and it appears that my trans had been seeping since 1964! This was not an uncommon problem in the era - even with cast iron, but especially with aluminum.
The repair was made by draining the transmission, then using a very tiny pick and file to open the hole up, and using a very small drill make the opening round and regular. Then I embedded a small screw and cut the top off. The whole area was cleaned with solvent, alcohol and finally detergent and water. The surface was roughed up slightly and the area covered with metal epoxy. Lots of time upside down with tiny tools and mirrors.
It all worked fine, so now I just have the normal engine oil seepage, which doesn't smell nearly as bad.
