top end.

Sounds like the spark advance. This bogs down at higher speeds in the distributor.

Quote from Haynes slant six manual:

"The ignition advance is controlled both mechanically and by a vacuum operated system."

There are weights in the distributor that move in and out due to centrifugal force, thus making spark advancements at higher speeds.

There is also the vacuum hose that connects to the carb and is responsible for spark advancements.

New distributor? If you have a Haynes shop manual for slant sixes, there is a whole chapter on the ignition and spark system.