A little more exprimenting and research....
I first tried filing down the bolster on the truck - that's what I did in the previous picture I posted. There are two problems with this. First, you need to fashion a custom bolster pin as the truck bolster is now too thing for stock MicroTrains pins, and the pin will interfere with the center axle. Second, if you want to relocate the bolster, you can't easily do it.
So I tried the other method - file down the body bolster and use unmodified (except for removing the couplers) trucks. This wasn't as fussy as I had feared, as the sides of the metal frame are hidden by the plastic casting for the end platforms.
But...as I looked at the one photo I have found of this car, I've become convinced that the trucks really should look inset from the ends. I realize that's not true of ALL similar depressed-center cars, but I think it is true of this one (which was apparently built on a GCS cast frame). The good news is that there is just enough room for the trucks to swing after lowering and substituting 28" ESM wheels (I track-tested it). It won't work with the stock MicroTrains 33" wheels.
So...on to finishing! Gotta strip and repaint it.