A lot of people seem to think that the unitized body construction on Mopars are weak and are flexible which is wrong. Even Dave Morgan who wrote DOOR SLAMMERS, The Chassis Book thinks they are as flexible as any car can be. It doesn't say that in his book but if you ask him at one of his seminars or elsewhere, he'll tell ya. He also thinks people with money are morons but that's all I'll say about that.
The factory built the body then hung everything on it AFTER it was welded up. The best way to check to make sure the body is square is to use a jig...or fixture for you PC folks
You can also use a smooth concrete floor IF you make sure it's flat before taking measurements but most floors will need some attention before it's good enough to use as a level place to work off of. If I'm going to do frame ties on a complete car, I'll put jack stands under the rear axle and then support the front end at the frame as near as I can to the motor mount area...and take measurements before jacking it up and again once it's in the air. If I see a difference, I look for why. You might see a difference with a convertible but they generally will twist and not have a bunch of sag if any. Another way to check if you have any sag is to check door gaps in several places before lifting and after setting it on stands.
The first time I encountered body sag or twist was when my dad had a flat tire on his 56 Plymouth Belvedere Sports Coupe. Once it was jacked up, the doors wouldn't open. I was only 7 at the time but remember it well....and of course that car was a full frame car. THOSE are the cars that have problems with sag and twist. Also cars with a full front subframe setups like Camaros etc and is where this type of talk comes from imo. Now if your car is rusty, treat it like a GM product....well, that's not fair lol. Having a chassis manual is nice because it has all the dimensions you need to make your checks to make sure your chassis is where it should be. Btw, the tolerance for our cars isn't exactly perfect either so don't knock yourself out trying to make it that way. I've found cars that had the body a good 1/2" different side to side....