Luckily, Kato has some of the caboose trucks in stock, so grab them. They don't always have them. But be aware that the mounting hole and bolster height are not quite the same as for the tender truck. But it should not be hard to file, shim, ream, etc, to make it work. The key element are those phosphor bronze inserts with the "thumbs" that go up inside the tender, and the wheelsets, and that should all fit. The side frames aren't the same either, so you might want to change both trucks so they will match.