I made a living towing heavy GN trailers in the '70s and '80s with 454 one ton DRW trucks like that one. They all had the TH400 tranny.
Folks simply didn't worry about tow ratings back then just as trucking doesn't now days.
I see my '90 chevy truck manual shows up to 13.5k lb tow rating with a 4.56 gears and around 10.5K with 4.10 gears.
As mentioned fuel mileage is gonna' be badz. I ran up to 28k gross combined with the carbureted version 454 DRW trucks at 4-6 mpgs...all day long at those weights.
The truck is one the beast pickups of that era.
