What size tires will fit on stock dart 15" wheels?

Your rim dimensions aren't bad. The 15x7's in the front are about right on, you can go a little wider with a little more backspace but you can't do much more because of clearance on the 15" rims. With a 15x7 and a 4.25" backspace you should be able to run 225/60/15's, although you might have to make some minor adjustments on your fender to bumper braces.

In the back you might want some more backspace for 15x8's. The one's you suggested aren't "bad", but they may not get you to your maximum tire size. But that depends on your rear axle. If you have a 7.25" rear, the +3 offset will be pretty good and you could probably run 245/60/15's. If you have an 8 3/4, you might want to go with a +6 offset, even with the SBP. Again, that should let you run a 245/60/15.

You mention the springs may have been moved? How much? Is it a 3/4" mopar spring offset?

Generally speaking, a 245/60/15 should fit most Dart's of that era with the stock spring location with careful backspacing. 255/60/15's are possible on some cars, but others need some trimming on the quarter lips to make those work. With a 3/4" Mopar spring offset (which is really only a 1/2" spring offset, just like Dr. Diff's) you can run 255's on pretty much any '70+ Dart, and most cars can clear a 275 with that offset. To get more than that you'd need the 3" relocation and a mini-tub.