Welcome. Here's are some pictures I've collected that depict the area you are working in.
Pic i: An explanation. If your rust is as bad as you think, the larger rear holes (1 one each side) are the most critical as that's where the rear shackles go.
Pic 2: A depiction of the crush plates. I don't know how accurate this is.
Pic 3: All the stuff that goes inside and outside for dual exhausts.
The welded-in nuts are for the dual exhaust shipping plates. If your goal is concourse, you need these.
Rear frame rails are available at National Parts Depot, Virginia Mustang (both companies are contributors to this forum) or Dynacorn
http://www.dynacorn.com/ I don't know if anyone of those carries the dual exhaust crush tubes.
Jim