I think I got it. Since the rockers are solid (and the drive shaft tunnel is intact), there's no flex/sagging when removing the frame member and floor pan. So, to do the job properly - change the floor pan vs patch, the frame member can be safely removed and was done. The floor pan is removed and the floor prepped. The frame member is then installed with a "repair" section that overlaps on the inside to make it strong (I'm assuming on both ends of the removed section), then it's welded to the floor pan with plug welds. Then the floor is welded in place and seam filled, ground, etc.
I have a 1974 Spider that's going to need floor pan work. The rust is minimal, but there are holes, so I'm not sure if I should change the entire pan or do a repair job. It's a low mileage, very original car, so I'm thinking it's best to change the pans.