that "plug" is sealed off
No it isn't.
It is not sealed. It's blocked from large acculmulations of rust and scale and other foreign matter.
As mentioned above, all SBC motors for all time, have had one drain plug on each side of the block. Yours isn't the only special one in the whole universe whose drain plug is not a drain plug. Believe that.
If you don't un-block it, the one bank of cyls will not drain.
Go rod it out with a coat hanger, or a drill bit if the coat hanger doesn't get it. It's a drain plug, make it drain, and get all the coolant out.
As far as rust removal, once you get the plug out and the block torn down including all the freeze plugs out of it (which by the way will enable you to see down through the water jacket to the drain plug holes), take it to the quarter car wash; blast the hell out of all the collant jackets, then put some CLR or something of that nature in them, wait an hour or so, then blast it some more; keep doing that until you can see metal around the cyls again.