If your tranny is stock and the shift tables are stock you are asking for a burnt tranny with that load... the converter and shift points in stock form will kill everything..
Not being able to get up to and keep speed points to a weak motor, keep lugging it and you'll burn it up.... As for what...
Water injection will help, a radiator spray system will help, real hood vents will definitely help... All depends on how much you want to spend and how much water you can carry, I have a 60gl tank just for this....
At what rpm are you running most of the time? If you are attempting to stay in the lower rpm band, say 2400 or less and are having to keep the throttle mashed you are creating a lot of heat, empty or loaded....
I watched your progress on the hummer thread.
I went with a hummer fan system in my dually, the hydraulics is tied into the steering system.
Anyway, my controller is adjustable for on & off temps and I tied into the original temp sensor in the d/s head, I'll use the p/s head location when I...
Have you checked codes? It sounds like the ecm might be trying to regulate boost and or fuel delivery, the ecm will derate fuel if it sees too much pressure.
Like this
https://www.intakehoses.com/cobra-head-elbow-4-id.html?gad_source=1&gclid=CjwKCAjwgdayBhBQEiwAXhMxtvWjK8cEa-a-Uvfs_6XDU0jYg2rWAcomjPgcCHgUBQM49QPNakHUAxoCgLsQAvD_BwE
I think if you post a few pics of your turbo with the stock filter box might help, side view, front view, etc.
Back when I did all this with my 92 I used a "cobra adapter" and it was simple. That air box was a flat style filter so yes it was a little different.