depending on your Fab skills, it is a pretty straight forward fix. only cut out sections you are going to fix at that time. You can use 1/16" plate to make new sections, make sure you cut far enough back so you are into good steel. with a little imagination, patience and a welder, you can do it.

The other option of course is to get a replacement. Call Rocket Resto in Olympia WA, they are getting ready to strip a 72 Challenger parts car and I am sure they would sell you the frame pieces.