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My mistake on the earlier post, I thought you had 315/35/18. It still would work to stay with your 305/35/18 Rears and drop to a 305/30/18 Front. You'd be going from a 1.14" stagger to a 1.20" stagger, that should be close enough for the computer and give you the 305mm width F & R that you want.
Be sure and give Chuck @ Corvettes of Westchester a call. A one time charge to remove the T/C & A/H nannies mgiht be the best way to go. Maybe even a mail order tune?