andric2 - So as we chase down evidence so far we have, if I've understood facts so far we have
Rear under seat dual exhaust supports
K code early style snubber bracket
History/evidence of panel replacement at least on the passenger side.
Use mounting holes on at least the drivers side read frame rail.
Can add other items as they are uncovered.
So a few more without trying to lead for specific outcomes
Where was the rear brake line mount located on the drivers side rear floor depression under the rear seat? Your earlier snubber picture shows the area but we have a shadow in one area and in another a may be left of a bracket (could be brake line or for a muffler) outward from there
Picture showing snubber and one of the support panels under the rear seat.
The seat area appears to be undisturbed at least the seams that are visible. Might want to try and collect at least the main floor stamping date. Believe since this is a convertible the rear floor section would likely be difficult to see but maybe from the bottom of the car if the area was cleaned. Just possibilities for more data