The correct question is, What is the proper distance between the mounting brackets on the radiator.
The shell flexes and while the distance will help you determine what is out of whack on the shell, the real need is a proper fit on the radiator.
In the end, it does not matter what the proper distances are, but what fits and looks right on the radiator you intend to use. So you really need to make the radiator right and then fit the shell to the radiator.
Keep in mind the radiator probably needs to be on the car with fenders and front splash apron as the bottom of the shell is over the apron.
Advice based on some lessons in hard knocks..