I just did this with Roadside Attractions for the upcoming DVD release. I actually put the footage side by side with the storyboards, as opposed to stacked on top which had left black space on the sides. I'm not sure why but the side-by-side version felt better. here's a look: http://www.arkhamfilms.com/images/roadside_attractions/grave_storyboard.png
Anyhow, scale your clips to the appropriate size and center them where you want to. For me the video scale was 59% & the image was 38%, it was scanned in at 150dpi.
