Have you got a buddy with an operating truck?
You could swap carbs, and then distributor.
Does it misfire under load, or only at idle?
Your mechanic is being logical.
You say it is misfiring on all 6 cylinders. That implies it has to be caused by something that all 6 cylinders have in common. That'd be the low voltage side of the distributor, the coil, and the fuel induction system.
regards,
Leon
