Given that he is experiencing problems with uni's overheating, incorrect pinion angle is one of the probable causes. Personally, (unless the car has large lowering blocks or is purely drag racing) i would say 6-7 deg differential is too large and would be more inclined to make it closer to 2-3 deg max. Easy way to diagnose is to place wedges in, these can be bought and added permanently when the correct angle is determined.