Back in the late 70s I was somewhat short on cash as a youngster interested in American OO but not short on ideas.
These were my first two locomotives. At that time my road was the freelanced Fall River Railway, running along the Fall River in Kansas. The boxcab was first, constructed on the frame of an AHM RS-2 and an Athearn F7-B body, somewhat in imitation of the homebuilt diesels that ran on the Rock Island.
The other is an AHM model (their Plymouth MDT Diesel) that is clearly overscale for HO and has got to be the easiest conversion there is; all you need to do is widen out the gauge on the existing axles and mount couplers at a new height. The model, marketed by AHM, was manufactured by Lima in Italy. The page of the 1979 AHM catalog that contains this model may be seen here in the HO Seeker website. I suspect the model is actually overscale for OO (!) but it looks very good with American OO models.
While I still use the little switcher as my standard for coupler height (specifically the rear coupler!), I rarely run either now as they are getting somewhat worn out. Lots of scale miles on this pair, pulling heavy loads, and they only have train-set quality drives.