Shouldn't there be a VIN stamped on the frame rails? If so, that should be able to be decided very easily and give you a lot of info on what it originally was.

Dave

1963 C10 Longbed Fleetside (1977-1987)
1962 C10 Shortbed Stepside (2003 - present)
2006 2500 Dodge Quadcab 4x4 (2007 - present)