If you are using a DVCPro HD or HDCam deck you can use the internal HD to SD output.
Most decks come with a SD - SDI output which has a live SD out at all times.
Then, you can use any of the SD 24P easy setups in FCP and capture through SDI.
If you are using an HDV deck then you should use the component HD out, but switch the
deck's own menu to do the downconversion (most of them have that feature) and capture via