How to Check Uni body for Straightness

I have issues in similar spots on my 67. I had just about the same holes on the bottom of the driver side rail and a poorly repaired torsion bar socket. I went with safe-t-cap caps for both pieces, the ones I got from them cover every area that your car seems to need. The people were super nice too. I have them preped and ready to install when it gets a few degrees warmer. The car is up on stands and I plan on running a straight edge as far forward as I can from rocker to rocker and find level, then as far back as I can, find level, and finally a straight edge across the two of them. A bunch of squares of different thickness plywood (underlayment, 1/2,3/4) and should be able to get pretty close, depending on grade of the floor. Figure after that make sure the radiator support is level and call it macaroni. Not sure if this is correct, but that is what I am currently doing