measuring for wheel and tire

A '72 Swinger with the stock suspension and no adaptors with a 15x7" rim? What rear axle? 8 3/4?

Typically, with the stock suspension you want pretty close to a "zero offset" rim on the Darts. For a 15x7" that would be a 4" backspace. If you have an 8 3/4 you can get away with a little more backspace than that, say 4.25". The 7.25" rears are a little narrower, so stick with 4" for them.

With the stock spring locations the biggest tire you'll fit in there is a 255/60/15. But of course every one of these cars is a little different, so measuring is still the best bet.