Chris,
I drew up a set of plans for the GC&E #12 for an HO scale model that was produced by Key Imports about 20 years ago. They are to 1/2" scale and are left and right elevations, top view and front and rear view, with some details of the rather unique firebox sub frame used on this engine. The GC&E #12 was built as a 150 ton 3 truck Shay and converted at Cass into a 4 truck model.
Since the WM #6 was a later version of the same class, dimensions for the trucks, cylinders and boiler are known. The tank dimensions were estimated from photos, so accuracy can't be guaranteed, but should be within a few inches. I donated a supply of blue line prints to the Mountain States Logging and Railway Historical Society for them to sell through their company store. I don't know if they have any left, but you might check their web site. If not, I may have a extra copy available.
Michael Allen