That hole in the intake manifold heat shield is there to allow oil splashed around the lifter valley to drain back out. There is no vacuum there, or under the intake at any point.
+1 on removing the top cover from the carburetor. Yes, you can actually leave everything else connected, except for the choke linkage, and run the engine to see if the bowl is filling properly. If it isn't, then you have an issue with the float, needle and/or seat. It's very common for the cover gasket to interfere with the float operation.
Yes, the plugged port on your carburetor should go to the vacuum advance canister on your distributor. Why don't you have one