Biggest Rear Tire Size

I'm sure this has been asked many times but I'm seeing many different answers. What is the biggest rear tires you can fit on a Dart Sport or Duster? My car is a 74 Dart Sport with SS springs and stock spring location with 15x10" wheels. I want to run M/T Sportsman S/R 28x12x15 tires. It looks like they would fit as the tires have a 10.5" tread width. Any opinions?