That's closer, but it's still not quite right. There is almost certainly an official diagram of the vehicle somewhere that is specifically intended for checking the (uni)body for straightness. These diagrams are simplified but have sufficient detail to point out measuring points intended for this purpose. They typically do involve the suspension peripherally, since places where suspension components mount to the unibody are often used as measuring points.

For example, on my 1989 Nissan 240SX, the innermost holes for the upper strut mount (part of the unibody) are referenced not only to each other, but also to the cowl and to the core support.

The core support measurements are among the most important in the vehicle because they inform the position of everything in the front end of the vehicle. And they are absolutely necessary because pretty much any significant damage to one side of the car is guaranteed to effect the other side in a unibody vehicle. This particular collision was front-end and it displaced BOTH sides of the core support and damaged both fenders - this is entirely typical of this type of accident and it is the reason why almost any car that has ever been involved in any kind of front-end collision that has required repair has poor fitment. Even running the bumper cover (in vehicles which have one) into too many curbs can cause this kind of damage, without actually cracking the bumper cover!

Anyway, moral of the story is that there is somewhere a document intended for this purpose. Measurements are included in collections possessed by auto body shops and if I am lucky I can get someone to fax me one Monday (or later.)

I am looking for something very much like the document you first suggested, but specifically applying to the unibody.