This may seem like a silly answer but have you looked inside the glove box door for a build sheet? There were pasted inside with a description and lots of part numbers in that era.
If it's a std. GM supplied 5 speed tranny in that day it is likely a Clark 282 or 285 series. The difference if I remember correctly was that one was a close ratio model, can't recall which one however. It is possibly a Spicer but these were much heavier built and usually not put into C60 series units. In any event there should be a mfg'rs tag on the transmission.
