As mentioned above I had the same problem earlier in the year. I ended up fixing it myself instead of replacing the bulk head and tunnel. I bet it looks something like this
I ended up buying some aluminum that was the same thickness as the bulkhead material that broke cutt it to the shape I needed and took it down and had it welded. I lapped this piece over the tunnel a lot more then AC did like 5 inches more. I am hard on chit so I realy beefed it up, I then ground the weld down flush and layed another sheet of aluminum on top of the one that was welded to the bulkhead and made one end flush with the front of the bulkhead and the other end flush with the other end of the plate below. Since my tunnel was cracked as well just like your buddies I proceded to take another sheet of aluminum same thickness and put it on the bottom side of the tunnel. Its hell for stought now and I can imagine it ever breaking again.
This was not a super hard project but takes some time. You have to stip the tank, steering post, and electrical crap that is bolted to the top of the tunnel. You then have to take out all of the rivets that will interfer with putting the plates in. Once you get the first sheet welded in you will have to drill from the bottom up to make all of your holes and then proceed to do this for the next plate on top and drill down for the last plate on the bottom. I bought new bolts for the electronics and used rivets in other areas like the corner of the plates where I felt I needed them. the bolt on the steering post will be long enought to do this. The brass looking piece with that the steering post bolts through that has the stops on it I used rouned fillup bolts that ened up sticking up a little to high so I had to grind the down a bit so the steering post would not rub on them.
I wish I had more pics but I don't think I took a lot, I will check. If your buddy has any questions I would be more then happy to help him out.