Do you have subframe connectors on the car? If not, I would start there.

Yes, frame connectors are present. Welded front and rear, but plan to weld them to the floors as well.

Topic update;


I've tackled the rust issues on the drivers side of the car, welded the rear floorboards to the frame connectors on both sides, and repaired and beefed up a rusted section of the rearmost crossmember.

Also noticed at the front, a connecting panel between framerail, firewall and inner fender wasn't spotwelded at all about 3" from the edge at the factory, so I rewelded this and added more welds along the edges.

Still have to fix the rust issues on the pass. rear sheetmetal but these aren't as severe as the left side of the car.

The car already feels way more solid while driving, which is no surpise of course seeing the extend of the rust it had;



















In the last pic you can see I still need to straighten and clean up the lines of the wheelopening edge a bit better.

Looks like typical old car stuff. As you do your repairs, are you putting in any anti rust treatments to the new and old exposed areas? Body cavity wax etc? Would be a good idea.

Now the outside is fixed I will work on rustprevention from the inside.
I've got a high quality liquid rubber that will be applied on the inside panel overlaps and welds. Then all this will be covered with regular body undercoating.

The car has gained a lot of structural strength again.
Before the rustrepairs, whenever I say, parallel parked on a crowned road, I could hear the car's rear body structure work.

But also when just driving around everything feels way more solid. I've even adjusted the rear shock absorbers one click looser.

Dan that design, will it require a shorter shock?