threaded opening between Cummins and Allyson Transmission

Sorry, not much of an idea about the leak unless the fuel filter(s) (or lines going to them) that are often behind the alt are leaking.
If they're in that area, I would clean off the filters and then run it to see if the fuel is coming from that area - you might even tighten them an 1/8 of a turn. If you use brake cleaner to clean up the whole area you can then sprinkle a little flour around so that the leak shows up easily.