You want enough support structure to keep the body form moving, but not so may that it inhibits your ability to get the job done! LOL

I'd recommend (at a minimum) use square tube from the front top corner to the rear top corner of the door opening on each side, AND a cross bar from the center of the driverside support to the center of the passenger support.

I've never done it myself, but have seen it done several times.

You can tell if it's straight by looking at the body lines and the gaps between the doors and fenders and quarters....assuming the car is still assembled. Otherwise you'll have to measure.