I didn't, & never would, suggest that you run the distributor directly to manifold vacuum. This is just dumb. I suggested that you use a Vacuum guage to measure the level of vacuum at various engine speeds, in cruise condition. I was speculating that there might not be enough vacuum, when dual Webbers are used, to take advantage of the vacuum advance function anyway. Gathering data with the vacuum guage was just the first step.
It would seem from your comments that there is enough to do the job, so question answered.