as the owner and driver of a 97 F250 former 460 gasser truck converted to a 96 P-pumped 12 valve rated at 235 HP from the factory, I can say that I have never looked back on my old 86 with the 6.9. My 96 Cummins is hooked to an NV5600 6 speed manual and the diffs are 3.55s. Its not often, but I have seen as high as 26 mpg. You can play with the cam fuel plate to get some free horsepower. I have never experienced a down hill runaway, but an aftermarket engine brake would always take care of that problem.
If power is a problem, I have seen these trucks with well over 600 HP be strretable and be able to tow
The main thing here is you remain all mechanical. Power is unlimited in these 12 valves, I have seen them make well over 1500 HP, so for you to make enough to pull a trailer should be no problem.
Good luck which ever you way you choose
Just sayin as I have been driving my Fummins for at least the past 8 years