[CGalbus]

I am getting a reading of about 6 in of HG on my engine and I am going through every possible thing that it could be. I partially covered the airhorn of the carb and the RPMs went up about 300 from idle. I have every port on the engine blocked off, I have sprayed starting fluid around the base of the carb and then I found out my intake bolts were loose and torqued them down to spec but even after putting them to proper spec the leak persists. My next thought is to replace the intake gaskets but I am uncertain if I should replace the valley pan gasket as well.

Is the valley pan gasket only responsible for keeping oil in or does it play a role in maintaining vacuum as well?

Can anyone share some wisdom on what I should try next?

[nytrainer]

Did you plug vacuum advance, PCV and (assuming) brake booster ports? If not try disconnecting and plugging one at a time. Might be hard to see but a cracked hose? or bad booster, etc. Worth a shot
Spray some carb cleaner around base of carb while running. Any change in idle would show possible warped base or bad gasket.
Also on my 68 the pass side rear carb bolt is hollow and a vacuum source. Needs to be plugged too.
Don’t forget the check vacuum port tree in the front of intake manifold.

[Red Box Rebel]

If you are running the Original Breaker Point ignition with a factory four speed with power brakes, you will have many vacuum hoses and vacuum ports. Vacuum is supplied through several openings in the manifold and carb base front, rear and sides.
The "Tree" nytrainer mentioned is actually called a "Ported Vacuum Switch." It has at least five ports that connect to hoses. The Vacuum ports open and close based on operating temperature.
The Four speed has a vacuum control valve at the back of the intake manifold and controls the advance and retard ports on the breaker point distributor. Lots of vacuum ports and hoses. Make sure the ported vacuum switch has complete integrity and the same for the control valve at the back of the engine on the passenger side.
If all the hoses and ports are sound, then you need to check the gaskets for the carb and the intake manifold.
nytrainer is correct that there is a hollow bolt at the right rear of the carb that is a source of vacuum for the vacuum control valve.
A bad valley pan gasket will possibly cause an oil leak, but not cause and engine vacuum leak.
After all this I assume that you have a stock cam?

[68gtoMN]

Actually, the switch at the front of the intake is called a Thermal Vacuum Switch (TVS), not a Ported Vacuum Switch - semantics. This operates via coolant temp

Low vacuum means a leak - duh. If your intake gaskets were loose for a while, then I'd think about pulling intake and replacing them as a wise step. Also, spraying Carb cleaner is a better choice than brake cleaner.

As mentioned above, check all of your old plastic valves (where hoses attach - like brake booster) and the ends of each vacuum hose as there might be dry rot and cracks there. Lastly is this a fixed headlamp or hidden headlamp car? If hidden, that is a vacuum system separate from the engine, but fed by engine. Many hoses there. :-)