Not only having 17 all the way around helps for rotating,but also the 17" tire is cheaper than the 18:. Just my .02 PS what is the biggest tire on a 17" up front w/out rubbing?

 

The 18" has a deeper inset. Remember inset... I remember when there had to be a couple of inches difference between the front and rear. If you go to the BFG or Nitto web sites they will give you the various dimensions. The only way you can be sure is to get some yard sticks and start measuring. I went with the 18s in the back because the lower shock mount on a 69 goes inside the larger rim but hits on the smaller ones.