It's not necessarily about what size tire will fit properly on an 8" rim (that can be an issue) but what width rim fits under an early A without heating, beating, trimming, and the like, without rubbing. A 7" wide rim with a 4"-4.5" backspace is all that will comfortably work.
Puzackly. I just confirmed what I've always thought is that the early A bodies (Valiants and Barracudas at least) have a little more room than the 67 and up Valiants and Darts.