Mike, I just looked in my "Model A Restoration Manual A-1" put out by Ford in 1955. Looking through the specs in the rear of the book it lists "TREAD" and simply says "STANDARD 56 inches".
On page 68 it lists the rear tire tread for the AA with duals and shows 55 1/2" center of one inside tire to the center of the other inside tire, then add 7 3/8" to the center of each outside tire. I wonder what time period this is for though, because the wheel offset did change at some point to add a little more space between the tires so they didn't rub each other when they met the pavement and spread out.