Stan,
The way I did it, without ever seeing how experienced "sleeving" gunsmiths do it, was to cut the barrels off at about 6". Then I turned a spud to fit the bore size, slid one barrel on it, tack welded it to it to drive it in the lathe, put the spud back in the lathe and indicated in the back of the chamber by bending the spud with a mallet, then bored the chamber out, repeat for the other chamber. That ensured the new oversized bores where the chambers were would be in line with the old bores, thus assureing the new barrels would align with the old barrels. It worked very well. I suspect the average gunsmith allows for alignment by leaving sufficient clearance between the new tube and the overbored chambers to allow a little wiggle room and room for solder. I TIG welded my 20-28g conversion, so I wanted a nice slip fit.
