I've always heard that a 225/60/15 series tire was about the biggest you could go on a 68-72 A-Body. In Jon's case even though he has 17" tires, the diameter of his tire is still smaller than your 15's. (26.1" vs 25.7")
A 225/60/15 is the same diameter as Jon's 245/45/17.
Here's the tire size calculator that I used.
http://www.miata.net/garage/tirecalcold.html