The 65-67 B body rear only buys you an extra 5/8" on your backspacing compared to the '68-70.
While that would mean you could run a factory 15x7, 4.25" backspace wheel with a 255/60/15, that's the best it'll get you.
Unless, of course, you run the ride height so that your quarter lip is a few inches above your tire. At which point you can run anything you want under 28" tall, because it'll just stick out past the quarter lip and you should have enough clearance that the rubber never hits the body. But that's not actually fitting the wheel inside the wheel well.
