My frame rails are extremely beat-up from years and years of being jacked up, left on stands, a collision with a ditch (requiring the frame to be straightened because of sudden oversteer after my dad did the 302 swap decades ago), and then more years of jacking up and driving hundreds of thousands of miles. Is there anything that can be done to restore them? You can hear the floor pans go "BUNK" when jacking up the front of the car from flexing.
If this is the case you need to get that car on a frame machine and get some frame connectors in it ASAP!
Connectors, I've been thinking about making some. I read- I believe it was in one of your posts some time ago-- that 2", greater than .180" thick box steel stick welded between the front and rear subframes does the job well?