FIX IT! I've had to do expansion plugs on both of my '55 C bodies. They're really quite EASY bro. Get several decent quality steel wire wheel brushes, and maybe a sanding drum, all of these the sort you can chuck into a decent drill motor.
Get the rusted old steel plug OUT! ALL of it! Use a flat head screw driver, pair of channel-locks, long-nosed pliers, hammer and chisel, et al to first remove as much of that mostly rust(ed) remainder, then use the sanding drum and steel wire brushes to make the surface of that expansion plug hole nice, shiny and smooth.
I use a 1.5" shop vac on these holes, combined with an air compressor, my fingers, a drain snake and/or fish tape, in short, anything I can to break loose the mineral deposits in the water jacket around the expansion hole, for as far as I can reach. You might be AMAZED with HOW MUCH CRAP can accumulate in the bottom of that cooling jacket after 55+ years of running water solutions around that hot cast iron block to cool it. Be warned!
NOW, resort to Chemical Warfare. You can cop a 1.5" rubber plug to temporarily plug that hole up while you use whatever witch's brew you favor to give your 413 an enema. Choose whats best for the sort of water most likely to have been in the engine, and the sorts of deposits you've observed. Plug the hole, invoke the Elder Gods, run your enema, drain. Repeat until clear and clean.
Get a DEEP BRASS PLUG, some Indian Head shellac,a large deep socket or big brass drift (better!) and tap that plug in. If this is a difficult position one, like, behind the motor mount, go buy an expandable copper plug. Otherwise, use brass.
Swab the rim of your expansion plug hole with a moderate layer of the shellac. This stuff WORKS, but you must give it a day to dry after getting the plug in. Tap the new brass socket, or use a wrench to expand the copper plug, making your plug snug in the hole. You should now coat around the edge of the plug, just covering the plug and where it meets the iron block with the shellac.
Take a day long break.
Fill your engine block with distilled water, start the engine and run it up to temperature. If you got that plug in straight and used the shellac wisely, your expansion plug should be good for at least another half century. Just pray the cylinders last. You might use your fingertips to feel around up on one or 2 of those while cleaning, but I digress. Brass plugs last longer, and work in better. Resort to the copper expansion plugs ONLY if rubber won't do and you CAN'T drive a brass plug in straight. You can always replace copper plugs later when/if the motor is removed. That makes things easier, when expansion plugs make trouble anyhow....