Thanks for posting the video. With vac that low it appears to be a big intake manifold or carb base gasket leak. An old trick often used is to spray carb cleaner around the base of the carb to see if the rpm goes up. Same can be said for the intake manifold. Be careful as it is flammable.
If the valves were not set correctly [too tight] it could be a valve issue.
If the heads were machined, [re-surfaced] there may be a geometry issue that prevents the manifold from sealing.
BTW, since it is a vacuum, a leak would just pull the soapy water in. ala water injection
Good luck...