What's My RearEnd???

I’m not too familiar with differentials. I’m putting a larger bolt pattern and power disk brake system in my 71 Valiant from a 74 Dart Swinger. The rear-end is quite a bit larger and I’m hot sure what it is.....8 3/4??? , 8 1/4??? or???? It has a 10 bolt cover and a front right side fill plug. The donor car had a 318 if that makes a difference. Can anyone help????

Thanks, Moparmadness

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