Unless it has been changed, the stock transmission would be a short tail shaft SM318. It has the 4 bolt side cover and a non syncro first gear. 66 was the first year for a full syncro Saganaw overdrive transmission to be offered for that truck. They are rather scarce and priced accordingly. The earlier factory overdrive, 55-65 is a SM319 is more readily available. It would also match the shift arms for your truck. Any overdrive change will require a driveshaft modification as they are longer than the stock transmission. There are electrical components and a pull cable required for each of these units to operate.
Fred
