Not sure on what bolt you are talking about as then i could think about the amount of space you have to do it.
If you can get an angle grinder there, buy a thin disk and cut a slot in it, then you can use a large screwdriver to unscrew it.
If that fails it is possible to do the same with a dremel but it will take you hours :shock: But yes i have done it with hundreds of bits later
Or the easiest way if you know someone who can weld, is to weld a nut onto the end via the hole in the middle then you can just stick a socket on.
I hate allen head bolts as there is always someone before you who has used cheap allen keys which dont fit properly :evil:
I always change any bolt that i can find if it even looks like its damaged, far easier in the long run