Well filing and scraping is sometimes in order for such problems, but if the gaps are big it's easier to get a new frizzen (from Jim Chambers Flintlocks) and fit it the correct way.

To do this, solder the frizzen down tight to the pan with low temp soft solder. Then use the existing hole as a drill guide to drill through the foot of the new frizzen. This will insure the best possible fit from a frizzen "as cast" When you have the hole drilled, reheat to drop the frizzen off and harden and temper the new frizzen.

To put the final touch to the fit, screw the new frizzen back on and scrape the pan so you get a dead tight fit after the hardening and tempering is done. Sometimes the heat treatment warps it a few thousands of an inch, so that's the reason you do it last. But if you do the job as I have outlined here, the amount of work is greatly reduced.

happy hunting
Steve Zihn