Trucks VIN number will be the legal towing limit, no Mods can change that.
You can increase the Ability to tow heavier loads but that won't change the legal aspect if you are ever weighed
Basically the weight limit for towing comes from Gross Combined Weight Rating(GCWR)
This has to do with brakes and power
And GVWR(gross vehicle weight rating) limit, this has to do with brakes and power as well as load IN/ON the truck.
If truck weights 3,200lbs and has a GVWR of 5,000lbs then you have 1,800lbs for cargo IN/ON the vehicle
Must include drivers weight in that, and passengers
Plus Cargo in the bed
And if pulling a trailer the TONGUE weight of the trailer, since that is weight IN/ON the truck.
Then there is the GCWR limit which would be the truck weight plus people and cargo and tongue weight and the difference would be the maximum trailer weight
So if GCWR is 6,500 and you have truck loaded weight of 4,500 then you could "legally" pull upto a 2,000lbs trailer.
So you need to get the GVWR(on drivers door sticker) and see if you can find the GCWR for the truck, Ford and some trailer places published that, by year and model.
Owners manual for that year will also have GCWR, by model, engine and trans
Automatics are better overall for towing, need a 2nd trans cooler but they are better, which is why they have a higher rating for weight.
I personally like manuals better but on paper(Science part) they fall short of automatics for towing weight.
People often blame this on "driver ability", that isn't even considered, the way automatics work is simply better for towing, science and physics, not a people problem, lol.
Yes frame can handle more than listed ratings but it will have a physical limit, but you have to reach that limit and then go passed it tell what that limit is, lol.
Ford only published legal weight limits not "breaking point" limits
V8 will certainly make towing easier but can't change the "legal" limits for the VIN
Even if you swap to automatic from manual, VIN is still manual so that's the limit
Depending on the state you might be able to get recertified as a custom vehicle and get a higher rating, but probably not cheap to do that
And just a heads up, insurance, if there is an accident, doesn't even have to involve weight or be your fault, if it is established your vehicle was over the limit, so legally should not have been on the road, your insurance company can deny claims, legally, leaving you on the hook