Here is a neat tool for such a thing.
It has an adjustable level. Where ever the car is at, you can put put this on a known straight part of the car, then set the level to it. You now have a tool to gauge other parts of the car.
Generally what you do is level or measure the "Box", that being the all four torque boxes on the body. This is on a unibody locations under each corner of the passenger compartment, usually where the front of the rear suspension mount, and the where the rear of the front suspension or sub frame mount. It is often a "jack point" like area near those mounting points. I can look under my Rambler and see the best place for you, my 59 has the same plateform under it as yours does I believe.