Kinda new at this. Worked with exhaust for aircraft, but this is a little different. It has rusted out sections near door on driver's side and behind driver floor mount. Used 16 gauge steel.
Join the club, pretty standard repair for the P15. If you haven't already check in over at the P15-D24 site http://www430.pair.com/p15d24/mopar_forum/index.php?referrerid=1152 there is a lot of knowledge on the old Mopars over there. What are your plans for your coupe? I have stuck with the flathead in my but jumped up to a 57 model 230. The last one I had back in the late 70's - early 80's had a 383/727.
I am going to stick with the original motor. My friend says we might need to replace the rings, but that might be all it needs to be a good runner.
Try plymouth doctor they make complete and partial floor pans plus almost every type of sheetmetal repair for the p15s..good luck