The Great White Brick - '69 Valiant
That's a tough one. I don't know. My Valiant still had/has good outer rocker panels, so we welded in new sills first with the car sitting on the rockers. That set the stage to remove the front frame and floor and get a new front frame in with the rockers and sills as a guide and a levelling benchmark.
I have seen a website with a guy and a Mustang where he hung the car from the ceiling of his garage by the A-pillars and C-pillars, levelled it that way, then cut out the whole bottom end of the car and replaced it. Maybe you could do that? Do you have any firewall or rear rocker parts to jack it up on?
I'd be careful not to cut out too much at once. Do it piecemeal, a bit at a time. Haste makes waste. Just think it through first.
In the end, if the car is a little crooked, the alignment guy will let you know and you can just shove it onto a frame straightener and tweak it a little. I expect the Valiant may need this once we're done. No biggie. It needed to be done to my '65 Barracuda too and now it drives straight as an arrow.