A lot of changes, more than I am qualified to comment on but… IMHO the frame would be rated close to the same as the OE frame, even though it was fish plated and double welded at the four joints it also still has a 22 inch longer WB… I am guessing the original WB was the 123 inch… a 31 ft. trailer would fit the 155” WB just fine as far as that goes… the old ¾ ton had a GVW of 6900 pounds and the 1 ton had a GVW of 7000 pounds on a 135”WB with a 9 foot bed… I think limiting your trailer weight to 7500 pounds or BELOW would be prudent and would max out the rear frame load assuming you have enough braking for the truck and it’s payload IMO, power no problem…
I wouldn’t worry about the th200 to much as long as it has good high capacity cooling and is driven like a TV and not a race car pulling a trailer, it will hold up pretty well…
I would not use OD on that tranny or the 400 while pulling the trailer, I don’t care how well it was built…