Paul,
Check the tapered pipe plugs on the back of the block.
I had a pesky leak on a C-Series Cummins in my motorhome after replacing the head. There was a 1/16" npt
oil gallery plug in the rear of the head which leaked in a steady stream when under pressure.
I pulled and replaced the plug numerous times with different sealers and it still leaked. I was concerned that the new head had a hairline crack at the hole, as the tighter the plug was torqued, the worse the leak. (If you've ever swapped heads on a large diesel in a pusher motorhome, you'll understand my reluctance to having to swap the head again.)
I finally got a 1/16" pipe tap, coated it with grease, and chased the threads. Then I flushed it out by cranking the engine over until I got a good flow of
oil out the hole and reinstalled the plug w/sealer. No more leak.
Good luck,