1968 NASCAR International Stock Car Racing Rule Book NASCAR Grand National Championship Circuit Section 20-6 -- Transmissions

a. Standard production, which are cataloged and available through regular dealer channels may be interchanged in any model.

b. All forward gears and reverse gear must be in working order.

c. Drive shaft and universals must be standard production only.

d. It is mandatory that a U-shaped bracket, no less than 2 inches wide and 1/4 inch thick, be placed around the drivehsaft and fastened to the floor or cross member of car to prevent driveshaft from becoming dislodged and dropping to race track.