I don't know if there are any differences between a '71 Super and a '76 standard with regards to stock rear fenders. Therefore, I can't imagine any problems using a 7-inch rim back there, and using 225's may be possible as well.
Again, I have a 6½-inch rim (et52) but I also have 8mm spacers. It may be determined by the offset you are using.
I spent a fair amount of time underneath my car measuring and dusting off some old math skills. I was 97% successful in the end. (I may try for that last 3% in the next 12 months.
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