I tried a similar tactic without the slots through the threads (great idea BTW) on an Italian bottom bracket where a thin Campy BB had been used in the frame for a long time and I replaced it with a thick BB and threads in the shell were pretty gunked up past the reach of the thin BB. I tried a wire brush, Dremel tool and solvents with no success. Unfortunately there was no BB cup I could use accept another thick Italian and I didn't have one to waste. I screwed the thick BB in and it pretty much stopped dead in it's tracks where the end of the thin BB would have stopped. I tried using a little more force to see if I could chase the threads, but I wasn't willing the sacrifice either the shell or BB. Fortunately a friend that owns an LBS (with he closed last month ) had an Italian BB die and for $20 he chased the threads (because it's Italian the tap went from the right side all the way through and came out the left side). My best to the OP, I hope he had more luck then I did..