Rear tire fitment with 3 inch relocate kit on 70 Dart

no ,its not mini-tubbed
If that's the case, the maximum tire size will be limited by the stock wheel tub. In a Dart, a 275/60/15 is about the maximum size you can go in a stock wheel tub unless the *** end of the car is sky high. Moving the spring 1/2" inboard would give you the same amount of space as moving them in the whole 3 inches if the car still has the stock wheel tubs.