Measure the length of the transmission and count the number of bolts on the side cover. I owned an un-modified 66 C-20 that I purchased from the original owner. It came with a 292 I-6 and the short housing Muncie 3 speed. That transmission is approximately 15" long has a non syncro first gear and has a 4 bolt side cover. It is likely the GMC used the same transmission. However, if the other transmission has a 7 bolt side cover and is about 22" long it is likely a Saganaw. You would have to modify your driveshaft to use it.
Fred
