Your truck is almost 50 years old. It was also made about the time GM started to switch over to the all synchro 3spd trans Muncie and Saginaw. Here is a quote from this thread at 67-72trucks, :"
IF it's got a 4-bolt side cover, it's a MUNCIE 3-speed, used from 55-65, in basically the same configuration n(X-frame cars used a shorter version, trucks bolted to the bellhousing differently). I don't know why, but a lot of guys refer to them as "Saginaws". The Saginaw however, came out in 66, and has a 7-bolt, flat bottom, side cover (both 3 and 4 speeds). There was a MUNCIE 3-speed used in 69-70 in some GM cars, but they are rare. A Borg-Warner 3-speed was also used in the "mid" 60's in some GM cars; it has a curved bottom side cover, they are rare also. Any manual trans with overdrive will have a solenoid, govenor, and separate shift lever on the extension housing as aprt of the O.D. Now, there was even a FORD Dearborn, top loader-style, 3-speed used in Pontiacs mid to late 60's (I guess to balance things out out the G.M. Hydra-matics used in some Lincolns in the early 50's). Pics are always helpful. Butch/56sedandelivery." Here is the thread at chevytalk;http://www.chevytalk.org/fusionbb/showtopic.php?tid/279792/
scroll through it there are pictures and casting numbers.