Do you have any pictures? There are some other subtle differences between a 67 and 68.

I think there's a model number/VIN stamped on the frame somewere. (Perhaps on one of the side frame rails.) I've never really looked for mine. It might give you a clue if your cab or body has been swapped out.

I agree, As it sits right now it's a 3/4 ton. Cabs are commonly swapped out on these trucks.