rear suspension options?

What is the distance from the outside of the tire at the widest point to the inside of the quarter lip?

255's are pretty much the absolute biggest tire you can fit on a Dart without a spring offset or trimming, and they don't fit without rubbing on every car. Plus the backspace has to be pretty much perfect. The MT ET street tires in 255/60/15 are listed as having a section width of 10.1" , while most '70+ Darts with the stock spring locations only have about 11" from the spring to the edge of the quarter lip, give or take about a 1/4" of body tolerance. So, even if your car is on the upper end of the body tolerance range you pretty much end up with the absolute minimum amount of clearance necessary to keep the tires from rubbing.

3.5" to the oil pan on a street car is not a recipe for success. I've run my cars at 3.5" from the ground to hard parts before, and on the street (around here at least) it's a royal pain to keep from dragging things with that amount of clearance, even with the significantly stiffer suspension I run on my cars. If you added a bump stop to keep the pan from hitting the ground you're also bottoming your front suspension out, which definitely isn't a good thing especially with the oil pan that low.

It's beginning to sound like what you need is a full on suspension rebuild, front and rear.