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DOT will see a problem and fine you to the point that you see it too.My personal opinion is that with swapping the running gear [axles] and using a 5th wheel, there would be no problem. As long as the 5th wheel is placed properly. Actually, if the 5th wheel was used on the regular sport trac running gear and there was brakes on all the trailer axles, I don't see a problem. Tractor/trailer units are short wheel base comparing tractor to trailer. However if you ever got in an accident you would be liable because of towing over the legal limit of your vin [like Capt. Ledd said ]. So don't get in a accident.
I added one word to Captain Ledds' post. What everyone is pointing out is that while it may be possible to increase the capacity it will not be legal. Unfortunately the government has no way of knowing your, or anyone's, skill level and has a blanket restriction on modifying the capacity of a vehicle. There may be legal ways around it, perhaps an engineers report, but that's probably more expensive than just buying a suitable vehicle. I've never checked what a engineers report would cost.Maybe we should rephrase our answers for better understanding:
No amount of parts swapping will increase your capacity. The Sport Trac's frame is insufficient to carry that amount of weight, regardless of engine, transmission, suspension, or axles. Sorry for being a bit harsh but there's no way around it. It simply won't work legally.
Again, as stated earlier, your only "option" is to body swap the Sport Trac onto the F250 chassis. Some states the title stays with the frame, some states it stays with the cab, others may want some other titling process, some may flat out say NO. Check your local laws.
The general consensus, and I tend to agree with it, is that with what you are proposing your frame will be the weak point. Since a 5th wheel hitch bolts to the frame, you are extremely likely to twist your frame the first time you have to make anything more than a gradual stop.Ok, I should probably refrase my question for better help. I'm looking to add 1-2k in towing cap. I have an 05 f250 that has been roled. I'm taking the running gear out(motor,trans,rear axle, and front outers) and putting them in my 05 sport Trac. Will it help with a 5th wheel? I'll be towing a nother sport Trac that will be modified for off road. It will be on a custom trailer made for this combo. The axle is a drw I'm taking the brakes over from the f250 and upgrading it as well. Can it safely to that extra 2000lbs?
Got the can off any way. Then I can make sure the motor/trans fits perfectly in there.I think boxing the frame might be easier and accomplish the same thing. Up to you. I'm not sure if the DMV has any authority on capacity, but I definitely don't know the legalities.
Good luck.