I hate it when people don't follow up and post the results.
So here are (is?) my results.
I was able to set up a small vise in the mini-mill, indexed to the table, and with it face off the front of the frame, the barrel forcing cone, the front of the arbor and the bottom lug where the barrel meets the frame.
They were all slightly out of square.
The gap is now 6 thou on one side of the cylinder and 5 thou on the other.
I will let the parts wear in a bit before touching it any more.
It came with such a slick action, that I feel it was worth the effort.
Thanks again to Doc Hoy for the suggestions.