Goofy idea....I would be curious if the problem could be corrected by shifting the rear end some on the leaf springs? Is that what you were talking about, Chryco?
It's been a while since I've done any alignments but the angle the rear axle makes to the centerline of the car (lenthwise) is called the Thrust angle. If the rear axle is not perpendicular to the centerline (90 degrees) then the car will not go straight down the street, aka "dog legging".
When you do an alignment, the Thrust angle is the first thing you check as the front end alignment and geometry is based on the relation of the rear axle to the front end in a RWD vehicle. That's what makes the car go straight down the road.
You measured from hub to hub, make sure the rear axle isn't shifted forward or backward on the leafsprings. Make sure it's not the components hooked up to the frame that are out of whack rather than the frame itself.
Can you take some measurements using locations on the frame instead of the hub areas? How are the door gaps, etc.? If the frame was off that much wouldn't the panels have some gap issues?
I wouldn't cut the car apart again, IMO....