I have a lot of 4-wheel cars on my 1974 narrow gauge, but they're either European standard gauge cars, or scratchbuilt, on Marklin chassis. If I was to use a Z ore car, and it would be practical, I'd keep the trucks. But then, mine is a "mainline" railroad, and I'm lazy.
Your design would be simple to 3D print. Make it a single piece, with a pin on top, and plug it into the existing body bolster. Since a long wheelbase car HAS to have some suspension, print a leaf spring at the end of the crosspiece, with the journal box under that. You could even include brake beams and shoes, and possibly a stub center sill filler and draft gear.
My mining company does use 4-wheel ore cars on the mainline, but they're rotary-dump gons, so wouldn't interest most narrow gauge modelers... They're rated for 25 tons, half of a 50-ton 8-wheel car.