Chris - In looking at your pictured wheel cylinders, I can't see what "port" you are trying to close up on them. I have an exploded brake diagram of your style setup below from a Chevrolet Truck Special Illustration book. The only added port plugs in that diagram appear to be the early style 2 piece bleeder valves. They were made up of a valve with a threaded hole for a 10-32x1/4 machine screw and washer to close the valve port. Later one piece bleeders (with the nipple for attaching a hose) have become the replacement for the old 2 piece valve.
EDIT: Mike B beat me to the post button.
Leaving my post up in case the picture helps.
