Tire/Wheel choice

I have a 73 Dart Sport 340 and am going to run the stock 14X5.5 rallye wheels with a 195/75/14 tires in the front. I also have two 15X7 (4.25 backspace) matching rallye rims, or two 15X8 (4.50 backspace) matching rallye rims to use in the rear. I am being told my best tire choices are either 255/60/15 or 295/50/15. I want the wheel/tire combo to be more towards the outside (near the fender) than stock and want them to fill up all that fender well room but I don't want to overdo it and have issues. Any opinions?? Thanks.