Did you ever find the source of your leak?
UPDATE:
Finally! I believe that I have found the source of the leak. After leaving the boat with a full tank of fuel for 25 days, I returned to find 21 ounces of diesel in the pan below the engine. The pan has valleys and peaks in the surface, so the fuel can drip in one spot and easily run to the other end of the engine. This doesn't help when you are trying to locate the source.
The only spot that I could find that was wet with fuel was at a TEE that connects the fuel return lines from cyl 1&3 and 2&4 and then connects to the injection pump and then from the pump there is a rubber hose to the tank. Wrapping a paper towel under that TEE showed that it became soaked with fuel after just an hour.
From this leak the fuel dripped onto a black vinyl tube that contained wiring and then followed the tube to the transmission end of the engine. This was key, because I could not figure out how the bell housing was wet with fuel when there was no fuel connections in that area. The pictures below show the vinyl wiring tube under the leaking TEE and then the path of the fuel to the starter pinion gear bump and the bell housing.
But finding the leak is just the first step in the fix. The TEE uses compression fittings (ferrule and nut) to attach the return lines. My experience tells me that just tightening doesn't often work (I tried, and it didn't help) , and since the ferrule is compressed into the tubing, you can't just put on a new ferrule. There isn't enough length of the return pipe to cut off the bad part and re-make the compression fitting, so I am at a quandary to figure out a proper repair. BTW, replacement return pipes are $200 each!
As a stop-gap temporary repair, I wrapped the TEE with Rubbaweld tape. It is an amalgamating tape that when tightly wrapped will bond to itself. Intended for wrapping rigging, it can also be used for temporary repairs to stop water hose leaks. There is no claim for fuel lines, but I figured I had nothing to lose by trying.
So far after 3 days the leak has stopped.
So for those of you that have read this far, if you have any ideas on a quality repair, please share.
@smokey73 suggested cutting the return line back to a straight section and then splicing in a new line made in the shape of the part removed with new compression fittings on all ends. Other than the difficulty in cutting the return line on the engine due to poor access, that sounds like the best solution I have heard.