Interesting.....
Giving a min or so of thought...... you might try to measure using a plumb bob....
1.) jack car up take rear tires off.... set car down on jackstands under axle tubes.
2.) drop plumb bob off frame rail in center of wheelwell - make a mark on the ground.
3.) drop a plumb bob off outside of wheel well... where the wheel opening molding is. - make a mark on the ground.
Now do this on the other side and compare the distance between the frame rail mark to the outside of the wheelwell mark.... if they are the same..... then the frame is centered under the body.
I need to do this to my car just to see.....
Let us know what you find out and how you did it....