Best way to increase Vacuum?

Not trying to high jack this, but with an A body, why power brakes?

A bodies don't need power brakes IF you use the proper master cylinder, with bores matched to the size of the rear wheel cylinders and the type of caliper (in this case you are using a single piston type '73-'76 caliper).

Also, Abodies are light enough to stop well without need for power assist.

Eliminating power boosters frees up a lot of space under the hood.

Once you eliminate the booster, you will not need to worry about vacuum for the power brake booster...just some thoughts...