You were advised to lightly polish high spots on the breech end. Lightly means just that as I learned on my first attempt at fitting an orphan set of Armor Steel L.C. Smith barrels to a OO grade with a blown barrel. Working slow and careful, I got the gun to close perfectly with proper clearance under the barrel flats, good bolting, top lever right, etc. My total cost for the blown gun and orphan barrels was about $100.00, and I was happy to make a cheap shooter. All was great until I found that I took a bit too much off the rim depth and some (most) ammunition with slightly thicker rims wouldn't permit the gun to close properly. I had made the mistake of stoning a few thousandths too much off of the breech end of the barrels instead of stoning the hook. Learn from my rookie mistake that you often can't put metal back once removed.
Fortunately, I was able to borrow a reamer to cut the rim depth a bit deeper on both barrels. I've used kerosene soot lamps, Dykem layout blue, Jarrows black, etc for spotting. And RWTF is right about a small straight acetylene flame making lots of soot. But one very cheap and readily available item for your arsenal is to hit up your wife for an old tube of lipstick. A thin coat on one surface will easily transfer to the opposing contact surface, and show high spots very well. Some acetone on a rag will clean it right off. I'm not so keen on this for inletting wood although some use it. I got this idea from one of the Brownell's Gunsmith Kinks books, which are a great source of tips and ideas.
