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Nismo OEM is 3.53. Aftermarket might be 4.08 IIRC. OP the diff should bolt right up. In case the input flange is a little different, like Sean said just swap over the flange from your current diff and it will drop right in.
You'll lose a little mpg on the highway since you'll be cruising at a higher rpm, but it's really not that much. The extra acceleration is worth it, IMO.