You can probably also use 50' cars as well. I know the Athearn cars work well on 18" radius. The 60' cars are hit or miss, some will, some won't. The ones I have the most problem with are the Genesis 60' cars. They're probably more prototypical, with prototypical truck swing.
Regarding your narrow wheels, perhaps measuring them with the NMRA gage would get them back into spec. If it's a truck problem (clearance), some judicious filing in the appropriate spots might help. But, I'd say try gaging the wheelsets first.
You might also loosen the truck mounting screw about a quarter turn,to give the truck a bit more play.
Kennedy