I think if nothing else, the dyno header tube diameters should try to be close to the same as whatever is going to be used with the engine. If the carb and ignition system that the dyno operator has is making more power than what is in the car, doesn't it make more sense to evaluate what is wrong with the running combination? Maybe there has been a problem all along that caused the owner to believe his engine was tired and needed freshening or upgrades, when all it needed was some better support parts.