Hy runningiron, the 40 steering box is a sector type, the 41 - 46 type is a recirculating ball type [less internal friction]. As far as differences between Chevrolet and G.M.C., my G.M.C. parts book lists a part number for the complete steering box assembly, while my Chevrolet parts book does not, however further research revealled that 41 - 46 Chev. and G.M.C. share the same steering shaft/ball nut assembly, so I will let you draw your own conclusions as to wheather or not they are interchangeable. I'm not sure as to the interchangeability of a 41 - 46 steering box into a 40 frame, but there may be some info on that here on the site if you do a search, hope that helps.
