I'm betting your three speed has 4 bolts on the side cover. That would make it a Muncie with non syncroed first gear. This was the standard three speed for trucks until 1968 behind six cylinder motors. It was around 66 or 67 that The "Saginaw" with 7 bolts on the side cover came standard behind 8 cyl motors and standard on all 1969 and later. The Muncies were built to handle the HP output of the early small blocks of the day, like the 283. I dont think I would trust it to stand up to a 259 HP crate motor.