Your best bet is to go with the Amfleet cars by Williams:
Or the MTH Railking Amfleet cars:
They are both 15" in length and can do O-31 curves. The MTH ones might look better aesthetically with Lionel's new Genesis locomotive (it was former MTH tooling anyway), but the simpler detailing from the Williams ones might go better with your Postwar stuff, if you decide to run it all together.