(In response to a comment: "Are you sure the wheelbase on the Bachman engine block is accurate for the Viewliner? The trucks just seem like they're really close together" )

When choosing a motor chassis and scale, I was mostly concerned with the truck spacing, since I based my scaling off of that. The Bachmann was also the smallest motor block I could find that had the best truck spacing. If I based the scaling off the wheel spacing and size, you are correct, the spacing is a little close. As long as I split the difference, I'm ok with the wheel spacing since this locomotive will only be 2.5" long and such differences will be hard to see that small.

Since the wheels are hidden pretty well with the cover plates and skirt plates on the prototype, I could probably cheat the look while keeping everything as accurate as possible. Probably could hide the bigger wheels and longer trucks as long as there's enough turning clearance for the flanges and skirts. Kinda did the same technique with Nature's Wonderland tenders in On30, hid them in the frame as much as possible despite being larger.

I originally looked at a KATO F7 unit which had perfect wheel diameter, truck length, and spacing for the trucks and wheels. The caveat of that is having to make the train larger in scale because of the motor block size and the train couldn't make the curves because of the car articulation (trying to keep the curves under 18" diameter).

Pardon the crappy photo, I literally put the chassis on the scanner glass! Drawing is a photo from a Van Eaton auction listing.