Fear! (Rust, not guns) How bad? What to do?

Hi gang,
Today been working around the interior of my 1972 T-E, also performing the weekly starting the engine ceremony (push one car to the back end of the garage, push the 911 with just the exhaust outside, after shifting trash cans and bicycles around) and just before quitting to have dinner and a glass or vino, I went under the starboard (right side, facing forward) and tapped a hammer a bit around where the square jack hole is located. I'd seen some cracks there and, sure enough, very soon I had a 2 inch=5cm cone of rust on the floor and a rusted-out hole just in back of the aperture for the jack. Being new to the "hobby" of restoring cars, I don't know whether this is the "rocker panel" or another part, can some one tell me? And ... how bad is this? And ... what else in that area should I pay attention to? The port side seems not as bad (but I didn't use the hammer on it yet), all around the car seems OK, the front bottom panel and suspension all looks fine.
Thanks for your ...support... sigh.


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Paul 1972 Zenith 2.2L