Did you get into the centrifugal timing advance system in the distributor below the breaker plate? Quite often, there's a spring missing or broken, and/or the whole setup is rusted solid so there's incorrect or nonexistent advance at certain engine speeds. "Out of sight- - -out of mind" happens pretty often, and centrifugal advance problems get ignored. The correct advance curve is essential to good performance, especially during part throttle operation.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!