Willowbilly3 is right about them Hyd Clutch Set up-s in the Older GMC Truck's . Most of are Med GMC Trucks at my work have them 1972-89 292 Str-6 and 4.3 V6 . The Bellhousing Looks Like the Old 1955-57 Checy Cast Iron one's . But it has two Threaded 5/16" Thread Bosses on the Right side with the 292 and the Left Side with the 4.3 . Ether Bellhousing would work on a SB . But they are Large and Heavy . The Clutch set up is a 12" Like a 1 Ton Truck Stock they had the three finger set-up and the 168 teeth Flywheel . The Clutch Master is just a Small two Bolt Mounting to the Firewall . Not a Bad why to add a 4-Speed ?