Schwinn often kept either full fillet-brazed frames or stamped headtubes around for years before building them into bikes. Since a Superior with a GJ (July '73) frame would have been actually painted and built up in '76 or later it should have a 4-digit stamp on the headbadge (assuming the badge is original) which will date the assembly of the bike to the ordinal day and year. I have documented Superiors with over 6 years difference between the frame and bike build dates. Post some pics of the bike and a close-up of the 4-digit number on the headbadge.