Check amazon.com for the book, Gunsmithing Black Powder Firearms (I think that's the title) by Sam Fadala. He has two books, one is specific to pistols and handguns, again I'm going from bad memory here. It has a chapter on fitting arbors. I used this method. Remove the arbor, weld up material on the end, turn it round again in a lathe, reinstall the arbor, then slide the barrel on to check fit. It should be too long to seat the barrel against the frame. Then file the end, a few strokes at a time, checking fit, until the bottom of the barrel shroud, where the locating pins are, just touches the frame. I put a slight bevel on the edge of the arbor because the bottom of the arbor hole in the barrel is concave, to give a good solid fit. I had a local machine shop do the welding and turning for about $20.
I've done this to an ASM 1861 Navy .36, that went from shooting a 2.0 inch group, slightly high, to shooting one ragged hole at POA at 15 yards. Also did it to an 1860 Army and saw about 50% inprovement in accuracy.
Get both Fadala books, they have lots of great info and better diagrams than I can describe here.
I hadn't though about shimming from the rear end, I guess that would work except that it would move the wedge slot, but most of those seem to have plenty of slop. Tuning the wedge is another topic that is critical to open top performance.