Stock suspension racers
I would say get an 8.8 out of a late 90's ford explorer. They come with 31 spline axles, posi unit, 3:73 or 4:10 and disk brakes and are almost the same width as a b body and should fit under the dart fine. Use a 15X8 and 6.5 backspace. Youshould be able to get a 275 on it.
You can pick up an 8.8 from the local you pull it junk yards for about $175 and you have everything listed above. Just buy some spring perch's for about $20 or cut and reweld the ones that are onit onto the bottom.
No need for clip eliminators on these. I ran one in my Mustang with 2 kits and never had an issue.