If the question is still, "what old diesel tows well," then I'll add my experiences with my 6.2L.
While recently pulling a 6500 lb load, 70 mph was easily maintained on flat ground, though I am simply uncomfortable doing that without a trailer brake. I chose to maintain 65. Hills weren't a problem, and I dipped down to 60 on the steepest one. I averaged 11.5 mpg with just over 12 as a high.
Specs:
Built 6.2L, MHITE06H turbo, DB2829 pump, 1800 psi injectors, Art-Carr TH700R4, 35" BFGs (truly 34"), and 4.88s.
I never pushed it past 1000 degrees pre-turbo and 13 psi of boost. That means that there's room left in the motor to haul more, faster.
Beyond that, there's room left for improvement in the motor. Add a larger fuel rate DB2 pump, 2100 psi injectors, an intercooler, and now we're talking.
