When trying to time the distributor, do you have the vacuum line disconnected snd plugged? Does it run okay while the vaccum line is off the distributor only to start running bad when you hook the vacuum bsck up?

Could you have the vacuum line for the distributor hooked to the wrong port on the carb? That line should only have vacuum when throttle is opened. When vacuum falls to nothing the centrifagul advsance should be taking over.

If it is a former boat engine, the cam may be a little more radical and cause some headaches.