Great work! Unfortunately, a project like this is beyond my skill level. The GP18 was owned by numerous Class 1 RRs including the B&O, IC, CNW, Lehigh Valley, MoPac, N&W, and not least, the B&M. It's on my wish list for MTH or Lionel to produce, but I am not optimistic that I will ever see it.
Please continue to share your progress photos, thanks!