Need some suspension/chassis guidance

When I mini tubbed my dart I ordered the wheels and tires, mounted then centered them left to right in the openings then measured how wide the rear end needed to be.

I made up a fixture out of two pieces of 1 1/2" angle that was about two feet longer than the car is wide with a piece of plate on each end so a floor jack could be used to hold it up. It was about 12" wide IIRC and made like a saddle the tires rested in. With the tires in it and a jack on each end I could move the tires in and out to get them where they needed to be and keep them relatively square with the body.

The Strange S-60 was within 1/16" of what I ordered. Also moved the rear end back 1" as I didn't want to cut the body.