Can I ask what makes you think your car is an original Z? Does it have the original paint?
The reason I ask is I am in a similar situation. My car has never been restored, but it is missing the original drive train. I go back and forth on this same question. My car will never be worth the same as a 100% factory original car, but at the same time I don't want the car to lose any more of its its factory identity. I am leaning toward the certification for this reason. I can take pictures, but the 3rd party credibility of Jerry might be better when I try to sell it. Will it add to the value? I don't know, but it won't hurt it. I'd be interested in knowing your thoughts.
I'm in the same boat as both of you. My 68 Z has its original paint under a coat of spray can primer but I have all of the original drive train except for the engine. I have the M21 with VIN stamp, 12 bolt, deep grove pulleys, Alternator, breaks, and other stuff. Just no engine block, I'm sure it is a Z/28 but how do I prove it once it is restored? It seems to me that Jerry's certification is a pretty sure way of proving it. Will it add value? I think it has to add some, how much I don't know. I feel this way because non-matching numbers cars that are real bring more money then a base models that have been cloned. If you don't think so than look a more then just Camaro's ever seen a Satellite cloned into a Road Runner, a real Road Runner that is non #'s matching will bring more then the clone. Same thing with a Shelby vs a Shelby clone, or Cornet vs Super Bee, Malibu vs Chevelle SS I could name more but I think you get the point. Think about tomorrow also, 10 or 15 years from now it might add value also to know that it was a real Z. just MHO