I bought a 68 Coronet a few years ago as a father/son project. The trunk was rotten and full of junk. I did look under it and the frame rails are solid. Well after my son cleaning it up and disassembling it last weekend. It appears the car was hit hard in the rear and improperly repaired. They hung new quarters and tail panel, pulled the rear rails and trunk floor back as far as they could and left a 1 1/2 inch gap between the trunk floor and rear crossmember and tail light panel They then just brazed a cover over the gap. It appears the repair was done very early in the cars life. My question is, All the tips I am finding talk about cars with rusty rails but good reference points. My rails are not rusty but are damaged from collision. Where do I measure from ? Another car ? I feel vey confident about getting them square from side to side I am most worried about up and down so that the rear cross member is the correct height in relationship to the rest of the car. There must be a simple way, test mount the quarters and new trunk floor and then start on the rails ?

I guess you could check with trunk lid in relation to the top of the 1/4s for gap and height difference. for being that far back on the car, being close is good enough for getting the fame rails strait, I wouldn't think it has to be perfect. just make sure everything looks 'correct' but measurments from any other 66-70 b-body wouldnt hurt

sounds like the rails are still buckled in somewhere? Is there a visible point of compression in the rails? The tail light panel attaches to the rear lip of the trunk floor, it's possible you could replace the trunk pan and have the quarters and trunk all line up but have the rear crossmember in far enough where you would have to shim the bumper to get the correct gap if the rails are still pushed in somewhere.

One last question, is the car driveable? I'd be curious to know if it tracks straight.

I was a frame technician at a Dealership, chances are if the gap is that big at the back, and the rails look ok, then the frame is bent over the axle. Check for crinkles. It would have to be pulled on a frame machine, if it was hit that hard, nothing else will have the strength. Do cross measurements underneath, to compare sides, using exhisting holes in the frame in front of the axle. Go from in front of the left rear tire to the right rear frame rail, and then do the opposite. ( In front of right tire to left rear rail.) Also measure the length of the rails to the axle, to see if they are the same or not. This way you can determine if it is bent above the axle, or just behind it, which could be repaired with a new frame rail. Hope this helps.