Monsiuer Hoof -
If you cannot locate the appropriate overly long drill and tap, two thoughts come to mind:
2) fabricate your own - we used to do this by silver-soldering drill-rod onto the end of the required drill bit. One uses a piece of angle-iron as a V-groove fixture to support the entire mess and keep it square and aligned. one can further sleeve the assembly with appropriate tubing if desired. If you were to use tubing, epoxy might even suffice for a one-use drill.
1) drive out the pin holding the cylinder arbor, remove the arbor for the needed clearance, and when re-installing the arbor, replace the pin with a screw
I am looking forward to seeing the results of your efforts esp, if you pursue the .41 Colt
yhs
Prof Marvel