can you get a ring spanner on the nut? or is it too far gone?
beating an open ended spanner will usually round the nuts alright!!

otherwise try a hacksaw, standard or junior and try to cut a split along the nut, or else just try to hacksaw the bolt off altogether. nasty place, not much room.
G

P.S.
could you use the open ended spanner as the brace? and crank it from the socket?