How did the car handle before dissassembly? (assuming youve driven it)

Had the alignment checked?

IF the car runs straight, handles good, gets god alignment specs, doesnt eat tires, and the repair isnt coming apart, assuming your using this as a driver, I wouldnt get too involved with rebuilding the repaired area.

IF youre returning the car to OEM for show (it appears its a real Road Runner) then getting the correct frame section is going to be needed. (clearly you know that part though)

I dont know if a repopr of the frame rail is made, but I would be tempted to get one off a clean southern/western car.

I agree about taking some measurements. Personally, I would take some from front wheel to back wheel - Taking them from the same side and across the car diagonally. I bent the frame on a Rmacharger plowing a number of years ago, and we used that method to find out if the truck was out of square (it was)

Thats a really odd place to have that sort of repair. Passenger side eliminates battery corrosion.....rust is a possibility, but how the heck would an accident damage that section? I know the K frame is a bit messed up, but thats a pretty well protected area. TRying to envision the accident that couldve caused it.