The Lionel replacement wheel sets are .145 tread width. I have a preference for .172 as they're more tolerant of the "gauge drift" of Gargraves track being used at the club. If your track gauging is good, the .145's should be fine.
On the issue of trucks, MTH (Atlas Premier) and Lionel 3-rail rolling stock tend not to have bolsters. Instead, trucks are typically mounted through the top of the chassis detail (this varies on Lionel). When I converted MTH center beam cars (also applies to MTH auto racks), I made bolsters from strip wood to support Weaver trucks. The later ones I did, I simply made a plug for the mounting hole from 3/8" dowel and drilled a 1/16" hole to support a 2-56x3/8" screw to mount the Weaver truck. I used the "bolster" section from the chassis as I had to relocated the trucks inboard on these early-issue MTH cars. MTH 2-rail trucks (which are still out there "in the wild") mount directly and are the right height on the later MTH cars with the Kadee mounts.
If you're brave and want to disassemble the newer MTH 3-rail trucks, you can use Intermountain 20050 33" wheelsets. Make sure you keep track of the insulated sides of the axles as they're single-insulated. The Intermountain wheels also fit Weaver and Athearn plastic trucks. The Atlas axles are too long and don't fit (don't know about the new narrow-framed Atlas trucks/axles).
