Its harder to run a 285-315 on 79-90 fox's due to smaller wheel wells, but you can still run them. You just have to masage the inner wheel wells, an roll the the outer fender lip. pending on the wheels backspacing is also a factor.
i run a 245-45-17 17x9 front, 315-35-17 on a 17x10.5 Rear but i alos have a 91 as well ...