On the wife's '67 I just installed a set of wheel vintiques (sp?) 15X8 Ralley wheels with 26.10 X 10 inch Mikey Thompson wheels. The back spacing of the Ralley wheel is 4-1/2 inches. There is approx. 1-3/4 inches of space from the tire to the springs that are in the stock location.
I believe this to be right about the maximum size considering the clearance left over that may not allow a 15 X 10 wheel because the tire is a little fatter than the wheel.
I can double check the clearance on my own '71 Duster if you like.
