The MTH trucks lower the newer cars to the correct height, but with some of the earlier ones, more drastic measures are required because they were from the "O-31 era" and ride high even with the MTH 2-rail trucks. The work-around for that is to insert a wooden dowel flush with the truck mount, drill a 1/16" hole and mount a Weaver roller bearing stuck (Intermountain Metal wheels) with a 2-56 screw. The other way is to remove the ends of the chassis and create new bolsters as in my first conversion below:
Original O-31 scale-length car (note the trucks at the ends rather than at the jack points.
New bolsters created and properly located:
Weaver trucks in place. Kadee height-check (needed to use a single shim)
Changed technique and used existing hole with the dowel and washers to better support the truck (you can't see it from the side of the cars)
The later ones all had the Kadee mounting holes (2mx8mm) and come out to the right height with the MTH 2-rail trucks.
Also, the Intermountain 33" scale wheels will fit into the MTH trucks pretty well.