Man... this sure isn't going easy for you. If you still want a gunsmith to open the throats you might give David Clements a try. Clements Custom Guns has a link here on Beartooth. David had done different jobs for me over the years (including cylinder throats) and he excellent work. The trick is catching him when he is not busy. Otherwise he does not do the little jobs by themselves. Just send him an email & he will get back to you.
Something else is worth mentioning. The average guy with some training can use a decent set of calipers and measure down to .001 +/- a bit. Working down to the .0001 +/- level and have the number be repeatable is out of the relm of most calipers and people. No disrespect intended but reading back through this thread, I don't know what size your cylinder throats are. That might be why David Clements had always had me send a couple sample bullets along with the cylinder

. All we really know for sure is the assembled rounds will not chamber and (like my SP-101) it's probably the perfect storm of bullet design and cylinder throats working against one another. So my humble suggestion is don't get tied up in the numbers as long as you get a good gunsmith (Bowen might do it too). Send him the bullet you want to shoot out of the gun. If you are going to do the project at home be sure to follow Marshall's instructions closely. I have never done it myself but Marshall is a pretty clever fellow.