Since my GTX had no rear seat belts when I bought the car, I'm guessing the outer belts bolted to the rear side panels and the inners bolted to the floor. The frame rails I found must be from a four door with the seat belt anchor tab welded to the frame. Is that correct?

I bought the outer wheelhouses probably nine years ago. The inners were just bought. To fill in the gap where the inner and outer wheelhouses meet at the side panel, I'm guessing the inner wheelhouse is the part that needs the longer flange to fill in that gap. Not too big of a deal to add some more metal to the flange, just some extra work. So far the used rails and AMD sheet metal are fitting together very well. The real test will be when it's time to fit the quarters.