Good Questions:
Metal work is expensive !
If you're a do-it-yourselfer at home with a mig welder then it might be feasible to go at the rusted parts of your frame rails and start fabbing patches and welding them in...a patch here...a patch there...etc...
Once you have a car sandblasted 100%, you'll soon realize just how many pins holes you have and just how thin the metal in your frame rails can actually be....then you have to ask yourself, how strong will patched up rails be?
Another reason to patch up rails might be if your the real anal type that wants to save every ounce of original metal possible.
In the case of my 71 Challenger, both rear rails would have required patches at the very back, near the torque boxes and also near the rear cross member. The car also needed a trunk floor and rear floor patches.
Keith Sim and I have done many, many rusted projects together...We have a method of calculating the cost of repairing vs the cost of replacing and nine times out of ten, it's cheaper to buy a new panel and replace it, then it would be to fabricate and patch up the original areas. With this much of the rear of the car needing work, the one piece DII panel was a no brainer.
The primer used is a BASF DP30 epoxy primer.
Yes it is an original NOS rear valance. It came with the car when I bought it..nice 70-71 piece.
Keith always butt welds panels with a Tig Welder...very little grinding needed.