My 74 Dart Sport rebuild

I've got the interior stripped out and the floor pans aren't too bad, just need patching in the front foot wells.





When I pulled the rear seat I found that the PO had screwed down aluminum flashing over the rear area and I feared the worst; got it all taken out and the floor is great, I'm kinda puzzled as to why they put it in.




The rear quarter on the driver side had previously been "repaired" by filling it in with spray foam and 1/4" to 1/2" bondo.




This is my biggest concern, the rear subframe is rusted very badly for the last 12"-15" including the rear spring hangers. I have a trunk that had been cut out of another car and the subframe is in great shape minus the driver side hanger. How big of a pain in the *** is this going to be to replace without the car "flexing"?