6 cylinder and 8 cylinder bellhousings look the same to the transmission in 1958. The transmission mounting pattern was used by gm for a long time... from about 1947 on in trucks, and after 1960 for cars. The sm420 truck transmissions were made by "Muncie"! A Muncie transmission such as the M21 and M22 has the same mounting pattern. Some transmissions may require using a different clutch disc.
