Maybe some credible info backed up with current dyno charts/ tuning / maintenance/ compression check/ inspection by reputable Z shop etc would help your case as far as value. As a buyer, I'd treat this car as a project that will potentially be in need of a lot of work/updating and would offer accordingly. Any kind of proof that this Z is healthy and well-built will help increase its value. It's only worth what someone is willing to pay. Have you had any legit offers on it? Best of luck