I've got 275/60/15's out back on my 73 dart stock spring location. Its a tight fit so it might work on some cars and not on others unless you do some minor fender work, the police rims have the perfect offset too. A 255/60/15 should fit without any chance of needing to mess with the fenders you just need the right rim with the right backspacing.
up front i run 245/60/15s and those are really close as well. If you want to play it safe get like a 225/60/15.
If your going for the wide rear tire look, going with thinner tires in the front will make your rears appear wider. when i had thinner tires on the front of my car my rears looked a lot wider. Now they don't look nearly as wide. just food for thought.