If you go the larger diameter tire route, have the tire shop get the hight and width dimensions first. Just asking for a 31" tire might end up getting you a tire that's too wide. Generally, if you ask for a 31" tire, they'll assume that you want a 31x10.50x15. That's a little too wide for an AD truck, and will rub on the fender. Any good tire shop will have a tire spec book that has the dimensions of the tires as they are mounted on a given width rim. You'll want to stay within 7"-9" to prevent tire rub. This is a lesser peoblem on '55 2nd and later, and the '67-72 can fit some pretty big tires in their fenders, 32x11.5x15 clears nicely on them.
