Mice built nest plugging drains. If I remember right, when I had fenders off I removed plugs(underneath) to vacuum out all the crap. (mainly my passanger seat padding)
I never watch a Riviera being assembled, but watched “B” and “C” cars being assembled at times(Flint). . I’m almost sure radiator support was on the frame and then fender assemblies from each side of the line. (I said almost sure)
Remember how these bodies were delivered to assembly plant. Doors were on (with skin) and aligned. Body was then dropped on to frame(with drive train). Radiator support then installed then fenders(with inners).
Try to save the original clips as the after market ones don’t seem to hold the molding as tight to sheet medal. You may want to remove back seat panel to release clips.