All good ideas above.
The joke from the engineers back then was "We are very precise with at have a lip that turns our measurements,+ or - 2 inches"
That statement was made by the father of the 426 Hemi Tom Hoover at a show many years ago.Thou exaggerated but not untrue.
All jokes aside CFD244 has a good idea. Do a search and maybe you can come up with frame specs for you make and model.
Frame shops had books with spots to take measurements from both front and rear sub frames on unit body Mopars.
Maybe look at the My Mopar site.
