The Ford heavy duty top loader 4 speed transmissions were, and may still be, the toughest and most reliable manual transmissions ever built.
How have they changed over the years?
The early (1963, 1964) top loaders had a small input shaft with 10 splines and a 15/16″ diameter. The output shaft on these early transmissions had 25 splines.
In 1965, these early 4 speeds were upgraded to a 28 spline output, while the input remained the same.
During the 1967-1968 era, Ford had to come up with a strong, bullet-proof 4 speed for NASCAR competition. This became the NASCAR Big Block 4 speed transmission and it had an input shaft with a 1 3/8″ diameter and a 31 spline output shaft. A select few of these transmissions were used in NASCAR grand national racing.
The NASCAR transmission was also used in 1969 and 1970 Boss 429 cars.