I can confirm that the original Ameriacn magnesiums are 15"x7" with 3.75" BS. Have been racing them for 40 years with 600-15 Goodyears. Just be aware 600-15 tires will still slightly rub on the rear springs and rear fender lips when you really push, even with a panard rod. On the fronts not much of an issue, but be carful backing up with the wheels at full lock.