Angle grinder won't fit else that'd be my first suggestion. Dremel with extension cable would probably work. It's hard to cut a nut off without cutting the bolt bolt also.
On that same job, I did cut through the one bolt that was hopelessly melded to the bracket for the shock mount. Less fun was the fact it was a washer head bolt so I had to cut through 20mm off hardened steel vs 8-10.
The shock itself was counter sunk no way to get in there with cutting tool.