That transmission is probably the SM-420, which was used about 3 days past forever in the 3/4 and 1-ton trucks, and several other applications. There will be a pipe plug about halfway up the passenger's side of the case that's both the fill point, and the level plug. Remove the plug, and, if necessary, add 90 weight gear oil until the level is even with the bottom of the hole. Reinstall the plug, and drive on.
Jerry
