Im currently trying to rebuild the rear end on my 75 clubman, its got the same problems as yours, but ive not even started yet. have you purchased a panel for the lower quarter panel, or made your own up?
The lower quarter panel corner was a shop bought one, it's a magnum panel from dsn classics, horrible e-coat if you can even call it that, if not fully removed affects weld badly and metal seems soft, makes it easier to shape but hopefully it is good enough quality, deffinitaly be plenty of wax oyl on inside, but this one has the lip that goes all way around, others stop at corner which is probably more correct if you have a inside corner! and I might have done it little different if realised inner corner was supposed to close of inner and lip go around, but corner I started on was completely gone and patched up badly. Anyway it still needed adjusting though, it has a lip that folds around to rear door aperture which needed re folding and the corner needed the lip cutting so it would bend around better, will still need a little filler for better shape to finish with. The lower valance worth getting genuine as better shape than none genuine, did try none genuine valance as genuine has more holes in than needed for this age and none genuine cheaper of course but just didn't match up to the lower valance corners!