I'm a bad influence. On a 67/68, a 285 absolutely, positively fits if you follow all of the custom backspacing tips and roll the wheel lips. 69's are easier since there is even more room. 285mm converts to 11.22" inches. I just measured a 69here at the shop and came up with just over 12" of clearance at the tightest point. That is at roughly 11 o'clock when facing the wheel. In my case, the lips have been rolled and painted with black 3M body Schutz paint.

How low your car sits and the stiffness of your rear suspension will make a difference if you can fit them in without rubbing.

I think the sizes you ordered will be fine with a 275-40-18 rear tire.