Now it's time to correct me if I'm wrong but do you need the bolt or the bracket that it butts up against to stop it from spinning,... because it sure looks like in the picture that part # 39-021 'is' the bracket that the head of the bolt would fit into.
OK,.. I went out and checked my car and now know what you mean,... there is a metal tab butting up against the head of the bolt, but it sure looks to me that it is welded to the actual bracket and is part of the engine mount bracket itself (part #39-021).
If it were me, I'd try to hold the square bolt head with a big adjustable wrench or vise grip when turning the big nut on the other side. It shouldn't be too seized up because you've already had it apart. If you can get that bolt off it sure looks like you'd have to replace the entire engine mount bracket or just the bolt with a normal head like rit14623 said.
That diagram is off a bit too,.. it shows a regular hex headed bolt for part #9YA09-1209 not the big square head that it is.
If that bolt is still tight I'd just leave it but if the square head is spinning, your gonna have a hard time getting it off regardless of what you try to do.