UPDATE!!
Felt like I was leaving everyone hanging. It took awhile to get it figured out. I went to a local diesel only guy after all of my other sources dried up several years ago when covid shut everything down. People just moved on.
Anyway, it was the return lines leaking under the plenum, or should I say plenum cover. The Exhaust Y is in good shape, no loose clamps either.
The downsides is, we also found a coolant leak from the head gaskets, very small, small enough to be of no concern to me not having to fill up the expansion bottle. Since we were already torn down, pulled heads...they were both pretty darned cracked too and replaced heads with the new-fangled thicker heads and got ARP studs. Upgraded the FICM to 53V by Bulletproof, but there's now a FICM issue where there was none before. BP is being great about it, but we're trying a rebuilt factory 48V version to see if it's the BP or something occurred during tear down. The mechanic is donating it at his cost as a tester for future 6-O client work. He'll have a good one to use as a tool. Harness appears in perfect shape, no open circuits or shorts in the harness.
I got lucky, caught all of this before getting myself stuck somewhere with my trailer. Also, with the heads off, the mechanic tells me the factory crosshatching on the cylinder walls is as good as new... that is a testament to my frequent oil changes and use of synthetics exclusively since I did my first oil change.
The cost of 2 new heads, new FICM, and all the miscellaneous parts, fuel lines repair, etc will be about $7500, to include some steering linkage work. I'm not happy about it but am happy that the truck will be better than factory refreshed when it's complete. I couldn't buy a new truck for close to this. We love this truck, so it is worth it for us to do this work.
Lastly, the mechanic did mention to me he hasn't seen one of these 6.0 is such good shape. My backside was smoking a little bit, but on the other hand, it isn't in poor condition.
Thanks for your time everyone. Appreciate the tips.