The gear ratio is pretty much determined by what you're going to be using the car for.
I have found 3.73s to be perfect for daily driver street/strip use. I have seen 4.10s at the track too, but they're just a little too steep in my opinion for normal street use.
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1998 Mustang GT
13.97@103
"Traction Impaired" or "Nitto Needy"