Northsea junkie hit on something - there is probably a thick aluminum bearing plate behind those bolts. It's probably corroded badly and should be replaced while you are in there. Before you spend too much time trying to work your way around the reinforcement castings, I would try to remove them from the beam (after removing the beam bolts and any rivets if there are any). It will make life a lot easier to have those out of the way. Expect that the beam has compressed a little where the main bolts are so it will take some effort to free them. An old-school body repair slide-hammer tied to the casting with a short piece of high tech line might help extract them.

You are going to need a little patience for this repair.