Ahh...well in that case the C/C plates will allow you more range, but you should still be able to properly align a stock suspension Mustang. You shouldn't need to drill holes to do so either. ANy chance the vehicle was in a front end collision? Worn suspension components like worn-balljoints and LCA inner bushings can allow enough movement in the suspension that may also prevent proper alignment.
One other thing. If the camber is off, the lower strut mount hole on the bottom where it connects to the spindle is slotted slightly. If you loosen up te two holes, you can push or pull the strut body in and out putting more or less angle on the spindle which can give you up to 1* of range for camber. Pushing the body in will add camber, and pulling it out will reduce the camber angle. You will need an alignment after this adjustment.