Hi Chuck,
Do not know of rubbers source.
Attached drawing is best I can do re chassis reinforcing for 850 Berlina body whether OT1600 or OT 2000 I am unaware but expect they were little different. The shaded areas show reinforcement at the front of the rear wheel arches, across at the top of the parcel shelf, and down the rear of the rear wheel arches ending at the engine mounts (inverted standard 850/600 alloy mounts by the appearance in most photos).
Hope this helps.
Regards Trevor