Rear of cab rusted, need a fix.

The rear quarters on our b2200 are rusting out on both sides of the back wall (otherwise the cab is rust free) and the sides of the cab under the extra cab windows on both sides were also kicked in years ago but a half assed job temporarily fixed the sides but now it's time to fix it all. Whats the best way to fix all this damage without getting a whole new cab? We can't seem to find one locally in better overall shape otherwise that's the route we would take. If we were to cut out and replace the rear quarters, where's the best place to cut them? Heres a couple of pictures to show the rust...