Manual steering box
You need to state the year. The size of the sector shaft changed ~1973.
Actually, we've had a lot of discussion about that on this site, but it appears that only the POWER steering box changed sector size. The A/B/E bodies supposedly used a small sector manual box throughout the run. Only C bodies and Vans had large sector manual boxes.
I'm taking apart a 74 Dart with manual steering so I'll be able to report on this more soon.
Here's great tech info on bigblockdart.com
http://www.bigblockdart.com/techpages/steeringbox.shtml