8 3/4 REAR - WHAT size brakes? 9" or 10-11"

i have a 8 3/4 rear end on my 1968 Barracuda one of my drum brake cylinders is leaking i went in to the parts store to buy a new cylinder but they told me that they had two in stock one for 9 in. brakes and another for 10-11 in. which one do i need? :burnout::burnout::burnout::burnout::burnout: