What I'd do is get the exact dia from "rollout" then go to the MP chart. then you'd have your dia and 3.73 ratio. someone here posted the chart that goes to the tenths of tooth count, I'd find it or see if someone will repost it. IE if says you need a 26.2 tooth pinion for your dia then find a 26 tooth pinion and if you need a 26.7 tooth pinion then find a 27 tooth pinion & if you have an early trans then yes you'll likely need the adapter