Almost any N scale with truck mounted couplers will RUN on 9 3/4 inch curves, if he really wants to run the longer cars. However, as the others said, long cars will look terrible on such curves, and shorter cars will also be much more reliable.
For what it's worth, Model Railroader has a page in the December issue on a new industrial switcher, built on a single SD40-2 truck. Minimum radius: 175 feet, or 11 1/8 inches in N scale. They don't say if that's the minimum for the truck, or the minimum coupled to another car, which is limited by coupler swing.