Inboard brakes like that are nice for low unsprung weight, but you need a much, much stronger drive shaft to prevent it from being sheared by the brakes. From what I understand, the average max torque a set of brakes can usually do is double the engine output (rule of thumb), and the instantaneous torque can be far higher due to pre-fade and/or stiction conditions.
And that doubled torque is what's coming out after the diff (geared down!).
Plus, I think he's looking for more of a drop-in solution
.
Sam