Butt welding followed by metal finishing and minimal or no filler produces the best repair. Skillful rust repairs can be made that only need high build primer. An auto body shop has to move jobs through quickly to make a profit, so you won't likely find an auto body shop who will make repairs that way. They tend to flange weld, get it close, and fill with plastic. Restoration work costs $60 per hour or higher. There are few shops doing this level of work.
Butt welded panels have to be carefully fit before welding. The time and skills involved make the process expensive. Flange welded panels just need some overlap, modest fit and then paved over with plastic. It depends on the level of restoration you want to achieve. It struck me you were after a high level of perfection when you said you wanted to do a body-off or frame-off restoration. If you don't want it as good as new, the frame doesn't need to come off. Did you get a chance to read Randy Ferguson's work?
