You should be fine with a 33 spline micro stub kit. I sold 2 used ones cheap not too long ago. You would be able to keep the trailing arms but would need a little fab work to mount the micro stub bearings. And then you would probably need new axles.
I had the gas 7.3 with a 10 speed. Towed my enclosed at around 11k pounds just fine. Gas mileage was in the 9s if there was no head wind. I did dump it and get the diesel 6.7. Mostly because I could tell my wife found out I had a small pee pee. The diesel is almost too much power for towing. If...
How much boost are you trying to run? For me with a dune car I’d keep my compression at 10.5:1. Especially if you’re not running 20lbs. No need to gut the motor down low to only pick up where you’re gonna have to shift anyways.