If the transmission is the one that came in the truck it would be a Clark 267V which was a relatively fast first and reverse gear and close ratio between 4th and 5th. It was made to split the first 3 gears then go from 4th low axle to 5th low axle and then back to 4th high axle and then 5th high axle. It was what was commonly referred to as a working 5th.