I'd be curious to see a 9mm conversion with a broken barrel pin! Sounds interesting, because I can't figure out why a pin would break. I agree a higher-up pin would be the best solution, and it would be more like a pin setup in a G3 or VZ-58 that way. A smaller pin would then be usable.
I have ordered a Shilen barrel blank and will use that instead of the Marlin barrel on my first build. It is HEAVY too. I'll soon be shipping it off to the smith with a big lathe