In the scene where Bonnie and Clyde come back for Buck, a close-up of Clyde reveals that the Ford Model A has a modern dash with a column shift overdrive automatic transmission which would have replaced the original 3-speed manual that the car would have been originally equipped with.
While talking to Frank Hamer in custody, Raymond Hamilton says that Barrow loves big block Fords and won't steal a car unless the car is powered by a V8. Throughout the miniseries, Barrow is usually seen driving a different Ford Model A that only had an I4 and is only seen once driving a Ford that came with a V8.