What type of rear end??

Yep, the mighty 741 case. This case has the smallest pinion shaft at 1 3/8" dia. although usually sufficient for A-body moderate horsepower applications. If you jack up the car and spin the wheels in neutral and they both turn the same way you also got yourself a suregrip. Then count the revs of the driveshaft during one rev of the wheel and you'll have your ratio. Could also look for the tags, but they are usually missing. Congrats on the Dart purchase, and I hope you get lucky with that rearend. Geof