I'm removing the EGR system components from a 305 ci and I'm trying to determine what type of threaded plugs to use to block the tubing ports in the exhaust manifolds.

thats not EGR, thats the AIR injection. the EGR has a valve located on the intake manifold and that allows exhuast gas to flow through the manifold into the intake runners. This allows the Nox levels in the exhuast to be lowered. The AIR injection is there to inject air right up to the back of the exhuast valves which prevents backfiring on deceleration and i forget something else.

-Anduh