15 inch tire suggestion

Hello,
Thought id drop in and start asking a few questions pertaining to my car. I just purchased a 72 Dart Swinger at the Nats and dont know a whole lot about them as far as what tire combination I can run on them. I am looking to run a 15inch tire but want to know what is the biggest size I can run all the way around. Id like to keep the tires all the same size. I have a set of Eagle GT's 245x60R 15's but not sure I can run them on all 4. any advice or suggestions would be appreicated. Thanx