I have an old Holley 750 vacuum secondary carb. on top of a hot 454, and there are extra ports on the carb. that don't connect to anything. One is a large threaded port near the bottom of the carb,. behind the electric choke setup, another is toward the top of the carb., just ahead of the choke setup which is the size of a vaccum hose, the only problem is I already have the vacuum advance and manifold vacuum hoses hooked up properly, and nothing fits these! Should I just plug them or what?

I don't know which one's for shure but the back big one I hook to pcv valve and one of the two smaller I hook to dist. the other I plug.

I already have the pcv and vacuum advance and manifold vacuum hooked up, these are extra ports for who knows what.

The large threaded port (behind the choke assy.) is probably an extra "direct manifold" vacuum port, if you don't need it, plug it.

You need to be a little more descriptive on the other small front one. Is it in the metering block above the mixture screw, or into the base plate?

The metering block one is ported vacuum, for the distributor advance, if it's in the baseplate, it's an additional "direct" vacuum for accessories.

The extra 'small' size vacuum port comes out just on top of the choke assembly. It does not come out of the metering block, that one I have hooked up to vac advance. When I got the car, this is the port that was hooked to vac advance, and the one coming out of the metering block was plugged. the one that is not being used now is also slightly larger than a vac hose fitting, a regular hose will fit but it is a stretch. It runs best with both of these ports plugged, but I was just curious what they were intended for.