A tri-glide isn't a converted 2 wheeler, the chassis was designed as a trike. I suppose you could salvage the engine, transmission, and front end if it is intact and buy a frame with a blown engine.
When they came out in 2009,thevTriGlide, and HD& Lehman together were doing them ...they were using the new 09 basic touring frame. It was a an HD /Lehman conversion. Theyre still made the same way...its just all done inhouse by HD now. Im pretty sure if the whole rearend is stripped off from the swingarm back you would have the same as a 2 wheeled tourer. I have an 09 TriGlide bought new when they came out and an 11 Limited bought new. Besides the raked trees on the TriGlide I dont see anywhere that the frames are different from the swingarm attachment forward.
I could be wrong but thats why I asked here as I know theres members that have been here since I have with 09s and have had theres pretty much apart such as Msocko etc....that would know them inside out.
The only difference that I'm aware of is the rear frame, suspension, brakes, left hand control, fork rake, exhaust, wiring to the rear, kickstand and the transmission mounts. Never really thought about it but it could be done.
A dealer I trust told me that the down tubes on the Trike are bigger than those on the 2 Wheelers. This is due to the 2 Wheelers needing to flex and the Trike does not.
On the 2012's, I can't see much difference from the swing arm forward. The mount for the kickstand is even on the frame. The front of the Tri is going to be raked more than on the Ultra.
As someone already said, it is probably going to cost more than it would be worth.