For $30K he should sell the '89, get a newer Vette and have you build up an engine for him. That way he gets the double thrill of a newer car with some extra Horsepower. I had a '85 Vette in '89 and the car did not communicate much what it was going to do near the limit and the ride was punishing, just not as refined as a newer one. I'm sure with new tires it would be better, but IMHO, it's just not a great car.
If he drives like a 18 YO, by the time it gets sideways in a corner (or maybe a straight) with throttle he might be off the road. You might not make as much on the deal, but you might keep a customer longer and he'd probably be happier.