I have a 59 3100 with a 261. I replaced the original 3 speed with a world class T-5 from a Camaro. Only modification to the trans was to enlarge the 4 mounting bolt holes to 1/2 inch. Changed the clutch disc to match the splines on the output shaft. It made a world of difference when driving on the highway, lowered the rpm's about 1,000 rpm's at 60 mph. I have the original 3:90 rear end in the truck.
