x2 beat me to it lol better use a decent extension not an asian one it might twist off....Try using a 3/8" drive extension. No socket, just the extension. Stick the extension in the center of the hollow bolt and you will feel it snap into place about 1/2" inside. That's what removed the bolt on my '74 Charger and every other small block I've taken apart since then.
The ext. will not fit. Vice grips will not grab and hold the nut. Almost time to break out the jack hammer...lol
Good idea....I`m going to try that.hmmm, thats looking ugly now LOL maybe you could get a chisel inbetween the nut and the base and flare up the corners and get the vise gripes back on it. either way i think you will be replacing it any way.