RBS stocks 9/16" or 5/8" cad plated steel freeze plugs that can be pressed into the stock GM bearing plate to retain gear oil. Most of the time the hole is 9/16" but at times the hole can be a little out of round and the easiest thing to do is drill out to 5/8" and install the next bigger size plug.
Most conversions take two freeze plugs per blower. Plugs can also be used in many blower snout driveshafts to plug the hole and keep oil in the front of the blower.
Plugs are sold each. Buyer can choose plug size from drop down menu.