Actually its the gears- yes the diffs are weak but the actual gears are also designed to meet the production spec of power and torque outputs. Kinetics Racing (who build Duratecs for the ex Speedvision Touring cup) say that their drivers on worked 2.5 motors with about 300 hp N/A would gun the car on the straights and peel the teeth off the gears in any gear they where in-
so you would need a new stronger diff and custom gears- IE $$$$$$$$$$$$
