Ron,
Your problem sounds more ignition related than mechanical or fuel. I used to have some problems at higher RPMs until I did away with the points distributor and went to an inductive one. Have never had a problem since. If you can get hold of a different distributor and try it that might be easier than starting to tear into the engine. Most point problems will occur at higher RPMs as that is where you need more voltage. Points have a tendency to bounce after a few runs. I am sure that you have already checked all of your connections and everything. You say the motor stumbles pretty badly at the 2000 - 4000 range but smoothes out at 4500 and above. A cam lobe or anything like that isn't going to go away at any RPM. I am not real familiar with the Duraspark setup but I once had a trigger lead in my Jacobs give me about the same type problem as it had worked loose in the connector.
Ron
