Te transmission is most likely the new process 4 speed overdrive based on the old A833 4 speed. It is essentially a four speed where 4th gear has been made an overdrive instead of 1:1 ratio. If it looks like a typical A833 4 speed, has an aluminum main case and one shift lever points up, the other down, it is an overdrive 4 speed.
Neat old truck, keep up the good work!