If you're married to having the widest steel rally wheel you can fit, I would say that you're likely best off calling up Wheel Vintiques and see if they'll custom size you a set for the best outcome.
I can't help you much with the fronts, but I will tell you that I managed to stuff 275/60R15's on the back of my '69 Nova using theseWheel Vintique's Style 30 wheel in 15x8 with 5" backspacing. It could use another 1/4 of an inch or so to be "perfectly" centered in the wheel well, but I didn't want to pay the huge difference it would have been to have custom wheels made.
I had to cut the dust covers off of the shocks in the factory location, and I do get some very minor rubbing on the quarter panels with the tires occasionally. Using Energy Suspension poly bushings in the leaf springs helps with that, immensely.