Just put the clip into your timeline and FCP will prompt the best settings for this.
First pick the DV50 -NTSC Anamorphic preset then change the timeline settings to match this below :
Frame size : 720x480 NTSC DV 3:2
Pixel Aspect Ratio : NTSC - CCIR601/DV... / Anamorphic
Field Dominance : Lower
Editing Timebase : 30
QT video settings
Compressor : DVCPRO50 - NTSC
or if you have a Matrox box you can pick teh Matrox DVCPRO50 - NTSC Anamorphic settings then match the editing timebase to 30.
or even though I never tried but AppleProRes could be an option as well.This codec is cool but in this case you loose some quality of your native files.But it might be faster to render.Maybe it's worth a try.