Dart Sport's are completely different than Dart's. The Sport body has an extra 1" of width in the stock wheel houses, same as the Demon/Duster and Barracuda's.
As far as "getting the wheel close to the outer fenderwell", if you get it closer than a 1/2" your tires will rub. You don't mention what rear axle you have or what bolt pattern, both of which are very important in determining backspacing because all the different A-body rear axles are actually different widths (7.25 vs 8.25 vs 8 3/4, sbp vs bbp). The rear axle and bolt pattern are as important as the size of the tire you plan to run in figuring backspace.