"Go not to the elves for advice, for they shall say both yes and no."
Yes, you can put MT N scale trucks on the Nn3 boxcar. Drill the bolster 5/64 to fit the new pin and the underframe works fine.
No, I wouldn't try it. You'll have to remove the steps, and even then, the journal boxes stick out past the sides of the car. It looks very silly. Most standard gauge cars of that period were about 9 feet wide, compared with 7.5 to 8 feet for the narrow gauge.
If it was me, I'd get some Evergreen scribed siding and build my own. Wooden freight cars are easy to build in plastic, and the ironwork can be made very easily with paper. Emboss the rivets and bolts with a dull straight pin before gluing and they look fine on the tracks. Larger bolts can be done with Grandt Line NBW castings, and truss rods are easier to add during construction than to retrofit to the MT cars.
One car you may want to look into is the frameless tank car from Republic Locomotive Works. These are based on the UTLX "Gramps" cars from the D&RGW, and the prototypes were standard gauge cars on narrow gauge trucks. They're probably a little too modern for the 1890s, but I believe they date from before 1920. Putting standard gauge trucks on them would be easy and prototypical.