The older cars are easy enough to convert if you're 2-railing them or setting them up with Kadees. I've done three (2-railed) and it's a pretty simple process. This is the first one I did, making my own bolsters and using Weaver plastic trucks. You remove the chassis detail from the car and shorten it by cutting out a section from each side. Making the bolsters takes time and you need to play with it to get the height right and prevent the car from leaning. If you're using the car empty, you definitely want to retain the steel weight plate under the car body.
The later conversions (2 cars). I cut out the original bolster from the chassis, inserted a 3/16 section of wood dowel and glued in washers as spacers to better support the trucks (you can't see them when the car's on the tracks, anyway). If you wanted to keep the car 3-rail, you can mount the trucks and secure the bolster with small screws rather than gluing them in place. You'd need to install Kadees or mount coupler bar extensions. I picked up the loads on eBay.
The empty car has two of the steel car weights underneath so it's pretty close to the weight of the loaded cars. They'll go through 36" radius (O-72) curves just fine.
The same procedure also works for the old auto carriers as well.