A diesel gmt400 is a touch easier to convert to a 6 BT than a gasser.
Mostly because the guage cluster runs through standalone sensors and wiring vice through the PCM.
No fuel pump to deal with in the tank, although I'm not completely sure I would trust the 3/8 fuel lines to feed a hotter 6BT with the hammer down.
Diesel trucks are "fly by wire" for throttle pedal, so you'd need a gasser pedal.
Diesel trucks have low stall torque converters than a gasser, but they're still a bit high for a 6bt so you'll end up changing that anyways. If you go 6bt and change the converter, make sure you get a triple disk lockup converter to five it a fighting chance. Billet front section will also help as well as being fully brazed.
Most you can "legally" get for a trailer rating on a gmt400 is 10,000 lbs. It has to be equipped with the 454 and 4.10 gearing for that. That's legally, not what it will actually pull. If you're ever challenged by transport on the rating, they're not going to care what you've done to it. They only care about the factory rating. That's just FYI info.
Even a stock/mild 6bt is over the torque rating for a 4L80E (oem rating 440 crank torque). Treat it nice if you use one or send it out for a round of hardened internals. Stay out of overdrive if towing heavy with a 6BT in front of a 4 l80e. The overdrive section is the weak link in the transmission.
A stock 4l80e will last behind a 6bt, but not like it will behind a 454 or 6.5. My 6.5 has dynoed 446 at the rear wheels (We've rough guessed it about 500 ftlbs crank) and my 448,000 km 4L80E is still alive. But I also have torque management active in my transmission calibration and I drive well aware of the fact I'm on a high mile stocker and well above its rating.
The problem isn't the horsepower for a 4L80e, its the torque. As a transmission builder to build one up for 1000 HP and he say no problem. Ask him to build one for 700-800 ft lbs and he'll laugh at you. Tell him its going into a 7000lb truck and he'll just point you to the door or say your warranty ends when it rolls off the lift....
98/99+ 4L80e's got some good improvemtns, including some revised and hardened internals. Most notably the input shaft.
4L85E is marginally better, but the duramx in the vans are derated/neutered because they have the 4L85E behind them. Its to make the transmission live the warranty period. Those dmax's make roughly what a "hot" 6.5 does....