Go with 275/60R15 tyres on 8" wide rim, 4.5" B/S. My '69 Fury I has 275's mounted on 7" wide cop rims (thanks Bryan and Ben) under the arse, and Ben said no clearance issues. I did a bit of research before ordering the 8" widened cop rims that will ultimately go under there, should present no problems either.
Please note though that the '69 rear axle is slightly narrower than the '70 axle (the '69 being a carry over from the '65-'68 cars)...