79 Cougar, What size tires should I use?

I have 79 Cougar (Same as 77-79 Thunderbird or Ranchero) and have bought a set 17"x8" wheels with a 5 3/4" offset. So I will probably have to get a set of 1 1/4" offset adapters. Can anyone tell me what what size 50 or 55 series tires I can put on it with or without the adapters? Yes, the wheels are from a Mustang, but the price was so good I could not pass them up.