Will they fit?

18x9.5" rims are ~10.5" wide outside to outside (the 9.5" is where the tire mounts on the inside of the lip). On my '71 Dart there's 11" from the springs to the quarter lip with the springs in the stock location. That means if the backspacing was perfect you'd only have a 1/4" of clearance on each side to the rim, without even considering the tire.

So, with the springs in the stock location those aren't going to fit. With a 1/2" spring offset you could probably run them, assuming the backspace is correct for your rear axle and brake set up.

But, without a 1/2" spring offset the widest tire you'll likely be able to fit out back is a 245, and a 245/45/18 or a 245/40/18 will both fit on an 18x8 anyway.