You're right about 463's boiler. The point is that they're all very old and subject to more problems in certification. Not that it can't be done, it just takes more money and time. The K-36 boilers are comparatively youngsters which makes them more desireale to restore.
I sort of agree with you on the fan appeal that 463 has but the K-37s can pull more cars so the challenge is there to which choose for rebuilding. There have been some movements toward private financing the rebuild of 463 but nothing has surfaced so far.
Les Clark