Two of the few US narrow gauge Mallets were Unitah 50 and 51.
This is a side tank saddle tanker that served a mining road and later a couple of timber roads. With the exception of the fact the Roundhouse 2-8-0 and 3 in 1 kits are outside frame locomotives they match up fairly well. I printed and did takeoffs from a Jon Davis drawing.
My ruling dimension for takeoffs is driver diameter which was known to be 42″ on the prototype. What follows is a takeoff table comparing HO dimensions converted from the drawing to actual dimensions from the model.
Note that overall length is almost identical as is the wheelbase between drivers in driver blocks. Driver diameter is a bit smaller on the model which will give it more of a hunkered down look. The wheelbase on the model is quite a bit longer but is adjustable due to bashing. The model makes up for the longer block to block wheelbase with a shorter front deck. As the wheelbase in the model is adjusted, room will be gained for a longer deck. Heights are close. The cab is longer on the model but needs to be bashed to add a door and bunker.
Distance from the rearmost driver to the rear truck and frontmost driver to the front truck are adjustable. The side tanks need to be scratch built so can be adjusted to fit the space. Overall, the model is close enough to the prototype for my purposes.
Here are some more prototype photos and in one case a LGB Unitah photo for details.
I will keep a builders log as I proceed with this project.