You're a cool Dad. You should be able to get a 8 1/2" tire in there, if you relocate the springs to the frame rail 10 1/2 to 11 depending on backspace of the wheel. I have 6 1/2 backspace and 10" slicks with about 3/8 clearance on the rolled fender. I still have 1/2" or so on the spring. You can shorten the axle use less backspace and load wider rubber. The 8 1/4 will hold anything he can throw at it with a 318 and LSD's are available.