Found a 1970 Dart Swinger, what's it worth?

On the center section you'll find a casting number ending in one of the following; 741, 742, 489. 489 is the later tapered pinion gear set, 741 is the small pinion gear set, and 742 is the large pinion gear set (if i remember right, I'm certain on the 489 detail). Any of these will be fine behind a street driven small block. To determine whether the axle is open lift the rear tires and attempt to turn them, if they spin opposite directions its an open axle, if they turn the same direction then its a sure grip. to determine the ratio, rotate the tire, one complete rotation and count the number of times the pinion turns, the ratio is the number of pinion revolutions to tire revolutions.