[Little]

I worked for Cummins during the '555 era'. All of my experience was with farm tractors,Versatile 555 4 wheel drive tractors. The engines had a high failure rate from various issues, however most managed to get to 4000 hr. I bought one of these with a seized engine (crank) and put a mechanical 8.3 in its place. Since there was room to move the rads ahead 5" there were no real fitment issues. The transmission failed about 2 yr later and I sold the tractor to another fellow who had a bad engine.
In our shop we have changed out 2 2003 Hummer H2's 6 litre gas engines to DuraMax 6.6. We used the LBZ engines and complete power trains from GMC 3500 trucks (2007) I had a GM mechanic blend the wiring harness and all gauges etc work great. After some fine tuning these are great vehicles which wife uses a a daily driver and we tow them behind our Wanderlodge.
The single most difficult part of the swap was the wiring harness. MrE has experience with wiring these older Newells.
If you are satisfied with the performance I would do an oil analysis and see what it says, maybe roll in some rod/main bearings and go. Good luck.