Every single time I help build a hot rod (on a car that hasn't been messed with) we always measure from the center of the rear wheel axle (using a long metal rod) to the center line of the front cross member. This gets the new front suspension (Mustang II or what ever) parallel to the rear axle tube. If the rear end is not square to the body then the front end won't be square either. I've never had an alignment issue with any car or hot rod that was built this way. I don't know how the factory did it but it you don't have datum points you can't get anything square or perpendicular. Frame alignment shops have books that show all the datum points on all vehicles, they use these datum point to repair/re-align car/truck frames.
My car drives straight and makes turns very nicely so something must be right!!
Treblig
