The shift rails have a series of notches in them, with spring loaded ball bearings that prevent the shafts from allowing more than one gear at a time from getting into mesh. With the shift box off and inverted, give the shift rails and the holes they slide through a good soaking with spray carburetor or brake cleaner, wait a day or two, then clamp the shift tower in a vise and try to move the shift lever. Don't get too energetic- - - -that top is cast iron and it could break with enough leverage.
Jerry
