Most carbs just have one power valve on the primary side.....but.....that is a pretty low vacuum reading for a relatively small cam. I would double check for vacuum leaks and increase the initial timing before spending money on any parts...JMO.
My problem is I have low idle because if I turn in the idle speed screw too much it is exposing too much of the transfer slot and thus not running off of the idle circuit. Vacuum is very good in all other ranges. Is a .508 small in a small block street car?