Nice job. I wonder if you'll be able to update the RS board to one from the latest GS class release. The one you're using has the incorrect whistle for a SP GS class. Lionel used some generic whistle in the last LEgact version from a few years ago.
I converted a Premier WP GS64 into a SP GS6 last year. Had no boards or wiring and was missing a few parts. Used a MTH PS3 kit and their GS class sound file has the correct SP "tug boat" whistle.