Medicdon,
I think an additional problem might be the diameter of the arbor. Early ASM arbors had a greater diameter than later examples. That being the case, replacing it with a later one would dictate a cylinder as well. Probably the cheapest thing to do would be to find a local welding shop to weld it up in place ( especially if you're not comfortable removing the arbor). If you have a vice, I'd chuck the arbor up in it and see if you can wiggle the frame. If you can't, proceed to the welding shop. If it wiggles, you need to remove and reinstall anyway. Either way you'll need the welding done first. After the repair, cleaning it up to allow the barrel assy to be installed will get you going. The welding shop may do the cleanup for you if you take the barrel along.
If you get that far, I'd fix the "short arbor" before you break it again.
If it was loose, you'll need to remove it, upset the shoulder material and torque it back in place. ( getting it welded up first will make this step easier).
Mike