Regarding the size of the K-37, several suggested that a standard gauge clearance gauge was necessary because of the size of the standard gauge C-41 from which they was made.
Well I was able to obtain a Westside K-37 made by Nakamura.
On my layout, I have several tunnel portals constructed using the NMRA HOn3 standards gauge. The actual portal width is 1/16” wider than the gauge and about 1” deep (before widening out within the tunnels.
There was was no interference problem and the widest part of the locomotive was due to the cab awnings.
note: if Blackstone ever makes their K-36 AND uses the K-27 cab awnings comparable to the large one used on the K-27s, that may become me the governing HOn3 clearance.
PS: I want the tunnel portals to be as small as practical so that they do not dominate the scene.