rim and tire sizing

Hey guys im pretty new to this whole scene and need help with figuring out what tire and rim combos will work with my 1970 dart also no tubbing and i have stock suspension
I like the way Toolmanmike has the 255's on his 70 Dart. I'm going to try and get them under my 68 GTS with Torque Thrust II's on it. I do have one of those measuring tools that bolts to the axle hub to check first tho.