I need a door jamb measurement

With the car being hit I'm sure you know to check the alignment of the frame and everything else too. It sounds like you've got the right idea, you'll want to tack weld the stuff you're fixing/replacing where you think it should be and then test fit all the body parts before you do the finish welding. The diagonal is a good idea, I should be able to pull one from my car and give the reference points. Anytime you can triangulate a few different points it improves the accuracy of the location. My car is actually fully assembled, but, most of the plastic interior panels on my car have the corners broken out of them so the pinch weld isn't covered. Pretty typical of the later cars.

If you don't have this from a factory manual it lists a bunch of the factory chassis measurements. It says Barracuda but they'd all be the same for any of the 108" wheelbase cars, which your Demon is as well. The diagonal across the bottom (59.9") sounds like a good one to check, that will tell you if the spring mount (and rocker) are in the right place. Of course all the diagonals would be good to check.
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