The easiest way to get it out would be a jack if you can manage to get some sort of extension on there at an angle.
If that fails you could drill out the rivets for the leaf spring mounting brackets to gain direct access to the bolt.
You could also center drill the head of the bolt from the top and then take a punch and hand sledge and drive the stud down and out.
Are you sure the heads of those bolts aren't spot welded?
There is 100 different ways to skin this cat, try some of them.
