First, does the 2002 rotor bolt pattern fit Devin's hub? If so, then you need to match the rotor diameter to your caliper position. If you're shorter than stock, then the caliper and pads will pinch less of the rotor. The reverse sounds to be your case though. The 320mm is dictates by caliper position. So as long as the rotor bolts ups, you should be just fine. If not, and you have to use the 310mm rotor for bolt pattern, you've got issues.
10mm is close to 1/2" and that is not a safe measurement to be off on brakes. I'd expect unpredictable braking with that scenario.