Are you taking the SD-SDI from the PDW and not the HD-SDI? The HD-SDI will always be HD and can't be changed.
If it's not cropped or letterboxed the SD-SDI output will look stretched in a 4:3 display or capture window because the video is still 16:9. This is determined by the setting of setup menu 930, which handles the aspect ratio settings used on the down converted output.
Otherwise check the 'Blackmagic PAL' preset you are using; PAL usually means SD and does not necessarily translate to HD @ 50i..