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I have an '81 and ran 255/60x15s on the rear and 235/60x15s on the front. They looked good. The front was 1/2 inch lower than the rear but you couldn't really tell the difference. I ran those tires with a rear sway bar and stock trailing arms had plenty of clearance. The car was somewhat lower overall from the original tires but lower is better. By the way, I ran Firestone Indy 500s and was very satisfied with their wear and performance. I am now running c5 wheels and KDW tires.
225-70 and 275-60 are almost the same diameter....
just some ideas....
If you don't want to add spacers the 255-60 is the better choice for the rear. I had to add a 1" spacer (adapter) to clear the spring and trailing arm, these 275-60s have a lot of sidewall flex and I wanted to be safe.... some members say their 275-60 tires fit without any mods, it seems every C3 is different....