what kind of engine are you runnin?

Your 7.5 does have 28, the traction lock put in the FX4 edition is the only one I know that has 31... (I'm sure that others will come and correct me if I'm wrong on that)

Anythin will work if you're willing to modify it... Just do yourself a favor and look for a 8.8 with a minimum of 3.73 LS, that way you won't regret your 7.5 purchase later if you decide to extensively upgrade your engine tranny...