Well, I tried my gripper sockets... no luck, it just popped off like a 6-sided socket.
Then I welded a bolt to the top of the nut... and the bolt twisted in half and sheared off.
Hammered the gripper socket over the top of the welding... that ruined the socket as it stripped.
Slapped a pair of vise grips on the welding, locked them so tight I nearly broke my hand when it finally clamped down... hit it with a rubber mallet, and *finally* it came loose.
Funny thing is, once the bolt cracked loose, it came out super easily. It wasn't cross-threaded or anything.
Anyway, I got my motor apart, and the pistons look fine, at least aside from all the coking on 'em from running so rich. However, the #4 piston leaves a tiny line of oil on the cylinder wall when I spin the crank, so that might indicate a ring-land or bad ring I think. I'm not going to split the block, I'll let Kspeed have at it now.