Inner panel how to deal with this rust?

Is this on a '72 swinger? The one in your avitar?
I had a '72 swinger that had the same problem as yours. It was a non vinyl top car and had vey little body rust. The guy i sold it to cut the whole corner (both sides) out of a '73 2 door scamp i had that was rust free in the window corners. He then cut out the affected area on the '72 and spliced in the repair pieces, first drilling out the spot welds and then installing it in layers. He used weld through primer throughout, and mig'd all of the joints. A very time consuming job, but he did it in a day.
The rust was about the same, and in the same areas except the rear. That was still rust free and clean.
Is this a '72 swinger thing? I haven't seen it on other years this bad even on rust buckets. Trunk floors have almost always been bad on the '72 a bodies that i have seen too.
I hope this helps.