This is a fairly common problem. I would try to not go for the perfect fill when fueling your Z. When the noozle clicks of stop adding fuel. I found that this works on my 96 Z(with the original sending unit). To replace the fuel sending unit requires the cutting of the carpet behind the passenger seat. Depending on the dealer this can be neatly done or butchered. Try stopping the fillup when it first clicks off. If that doesn't work let the dealer go at it, with a warning that you won't accept a butchered repair.