I've only got 900 miles on the Cummins 5.9L 12-valve that I swapped into my bobbed deuce. The only casualty so far was a Horton fan clutch, but eBay came to the rescue. If you poked me with a stick until I told you a negative to the swap it would probably be the Allison AT545 transmission's 4th gear ratio (1:1). The only 2 ill effects of running the Cummins on the governor for 4 hours coming back from getting the deuce painted: fuel mileage and I burned the paint off the exhaust manifold/bottom of downpipe. I would call that a win.
Freightliner engineered the powertrain package from my bus donor vehicle for the deuce engine swap to 30k GVWR, so it has no issues toting around a 12,500 lb bobbed deuce without a trailer. And I don't actually check my fuel mileage, but I'm sure running the Cummins on the governor would be a lot less economical than at the sweet spot of 1800 rpm.
Would I do it again? You **** right I would!