Hy GarlandFarms, Spanky is our resident C.O.E. expert, but I think that that cabover body should mount on you Chevrolet Chassis. The GMC engine is a bit longer than the Chevy and would normally take a different front cross member (which you would have if you bought that GMC body/frame piece. If you could post the serial number or at least the series that would give us more information. Any pieces that might be different between Chev and GMC for mounting the body on your chassis would be on the frame piece that you would get with the GMC body, hope that helps.
