No idea where to start

Rusted floors have been patched with old lisense plates and pop rivets, for years.
Most common had to be under the back seat of a VW Beetle were the battery was located.
Today we can buy stamped repop sections for our A-bodies.
Patch yours anyway you choose.
The main thing to do is fix the water leak. Could have been the heater core leaking but my money is on the cowl drains clogged and/or that vent box to cowl seal. The left and right seals are made different and the right is more prone to failure.