All things equal, a cab and chassis should wieght less as you note, no bed on it.
BUT with that said, in the past, some C&C frames were a bit thicker in metal. So potential is there to wieght more as it sits.
A nice thing about C&C trucks, is they are easier to get max door sticker GVWR! A proper licensed body manufacture can change tires, springs, etc. Make the gvwr be equal to the axle ratings. A bit harder with a completed rig from the factory.
An 8' 1200 lb bed is on the heavy side even for a flatbed. I also don't know what's included in that wieght. It could be on par for what it is too.
Marty