That bolt is just a cover, it doesn't do anything. Unless, of course, you want to push down on the adjuster rod to help the spring along.
It isn't a normal bolt, anyway. You could get away with a normal bolt and washer if necessary.
So just use any method you like to get it off if you need to, visegrips comes to mind. Obviously it will be a bugger if the engine is in the frame. Maybe you can hacksaw a slot and use a big screwdriver or impact driver?