you can pick up RB motors for the best price at www.ace-hobbies.net and the shimming is inside your case,when you pop off the covers from the front and rear diff cases, the diffs them selves are shimmed inside the case with small brass shims. Basically you make sure that the diffs them selves are not allowed any play with the pinion gear otherwise the power of a stronger engine will strip the crap out of them really quick. Should be in your owners manual for the car.