So very stuck...

If you are making 400 hp with 9:1 CR, engine must have a sizeable cam in it. That means less idle & cruise vacuum.
So trying to get a stock VA to 'work' could be an exercise in futility. Buy a cheap Chinese HEI dist, which comes an adj VA unit that has a softer spring [ Have used heaps of these, zero problems, can buy here for under $100; new adj VA unit costs almost as much & all you get is a VA unit....].
Adjust VA Allen Key fully CW [ softest spring setting ]. You do NOT use this spring to limit the amount of VA applied. That will result in erratic operation. The amount of VA is controlled by the how far the actuator moves [ it has about 30* total ]. With the HEI design, it is very easy to make a stop that fits under the mounting screw to limit plunger travel.
For the centri curve, the weights/springs are under the rotor & easy to modify.