Here is a B body rear, throw a suregrip and whatever gear you want in it, move the perches in and bolt it in. You could potentially find a complete center section to bolt in already set up with a suregrip. You could probably get this done for around 6-7 hundred, if you are patient and shop around.
BTW, IF you consider it I would offer 200 for it. I paid 100 for mine complete, added all new parts, suregrip and new gears, I put it together for just a shade over 600.
You will like the way it fits into your Dart Sport, it will bring the rear tires out to match the front width.
