Measure the distance between the mounting ears of the bracket, next measure the length of the mounting foot of the alternator and deduct it from the “ears” measurement. The remainder is the length of the spacer, which can be cut from steel, brass, or aluminum stock, and center drilled.
Can't understand why you're breaking those bolts, they hardly have any loading. You can also drill out to 3/8″ and use a grade 5 3/8″ bolt.