You will need a 50 oz imbalance flywheel. All foxbody mustangs that are 82 or newer have a 50 oz imbalance. Whether you should go with an aluminum or steel flywheel is another question. Aluminum will allow the motor to rev up faster because it has less rotational mass. However, this means it also has less rotational inertia. This will cause a lot of clutch chatter on a stock motor and stock rear end gear and make it rather unpleasant to drive on the street. I would go with a billet steel flywheel unless you are making a lot of torque and have a tall rear end gear eg 4.10.