100k, should still have lots of life in it.
What gears? Is he running oversized tires?
Those alone can really affect how well a truck will tow.
If your in hilly/mountainous area 410s would be the minimum if not 456 especially with larger tires
Is it going through oil between changes?
If it is, plenum gasket under the kegger intake is more than likely leaking. (poor design/prone to failure)
Another area prone to failure are the heads. Tend to crack between valve seats. If its got ping/detonation while towing, could also be the heads.
If you want 300hp/350trb (crank) could get 50mm tb(thefastman.com), diy on the kegger (w/hughes plenum kit) as well as some 1.7rr on top of the Hughes(EQ) HD 1.92 heads.