It looks like the diagonal arm (DA) is supported by the bolt, and that the boss at the inner end just helps to keep things centered but mostly gives the bolt somewhere to thread into and bottom out. There is enough material left on the boss that it should hold things in place as the diagonal arm bushing will be forced against the frame side by the bolt and two washers (on the outside) and the bolt itself and won't move around as it did before. Scope out the bolt really closely (use a metal straight-edge) to make sure it is not worn along its' length. If it is, get a new one - maybe get a new pair anyway..... They're cheap.
Looking at the replacement brackets shown by Greg, all they have is a big, honkin hex nut so those don't do a whole lot to center things - they're depending on the bolt and it's close tolerance with the inside of the DA bushing to holding everything in place, especially with the DA inside bushing properly snugged up against the frame.
That said, it wouldn't hurt to build up the boss surface on the frame and then Dremel it off nice so the DA bushing fits nicely over it, but I'm not convinced that it would gain you a lot unless 3/4's of the entire boss is missing. Again, the bushing is close against the frame and the bolt will hold it from lateral movement.
If it were mine, however, I would get a couple of sets of DA bushings, 4 new washers and a couple of new bolts, press new bushings into the DAs and put it all back together and not even think about it.
Yes, it takes a lot of snot to break those bolts loose. If you look closely at the side of the bolt head you'll see a small groove in it. Once you reassemble everything, torque that bolt up to 90 ft. lbs. (dry, no locktite, no anti-seize compound), then find where that groove is and, with a small cold chisel, crimp the collar of the mounting bracket into that groove to prevent the bolt from loosening up (that's why it was so hard to break it loose when you took it apart!) Don't scrimp on the torque: 87 - 90 ft. lbs. Use a torque wrench.