Don't think there would be much issue on the inner fenders. Think main thing would be supporting the frame itself, on solid surface. Frame, front and aft, and main body shell are so-called unibody, so should be fairly good, as long as it's kept together.
Problems with the inners?
- - - Updated - - -
For what it's worth...I had to replace my full '64 floor pan, using front/rear pans. Time to get a good spot weld cutter!
I studied how the pan was originally put in, and put the new pans in same way. Drilled small pilot holes through each, lol, spot weld, then used the cutter, only on top plate. Once all done, pans out, clean up the mess.
Only 'spot' welded new pans in, using the pilot holes as guides. Finished by using original type seam sealer, like the factory used.
- - - Updated - - -
Something else I just remembered. Before any welding was done, I lined up the pans as wanted (I went wall-to-wall), and used #12 sheet metal screws, to lock it all into place. Then started spot welding. Worked for me, and kept it all in place. Easy enough to pull those screws, and plug the holes, after pans were tacked in.
