When you are at full throttle there is no vacuum, so there is no vacuum advance so it can't affect full power. Large trucks and boats do not run vacuum adv due to the nature of how they run.
They are either at idle or under a heavy load where there is no vacuum. Ported vs manifold vacuum Ported vacuum is applied above the throttle plate in the carb so it only has vacuum as you start to add rpm.
Leave your trans hooked up to the full/manifold vac source on the carb baseplate. Hook your HEI up to the ported (part-time) vac port on the. Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air In most applications the rotation rate is set by the application ( engine speed in a vehicle or machinery speed in other applications). the car up a hill) then engine speed is limited by the load and minimal vacuum will be created.