Hi Roy,
As a professional appraiser, I’ve run into similar scenarios, but haven’t appraised a Grand Sport yet (though I did get to touch 003 back when Tom Armstrong owned it).
In the case of a super accurate recreation, I think you’re best served by looking at similar efforts in recreating Birdcage Maseratis, C and D-type Jaguars, and 250GT Ferraris using the running gear of other 250’s. These more accurately represent the effort and complexity as opposed to using examples of more plentiful and simpler Cobra recreations.
One thing I’ve seen that seems to impact the value of these recreations quite a bit is the ability of the car to be used in vintage racing or touring events like an original car. This really affects the buyers ability to enjoy the car in certain ways and use it in the company of other collectors.
With any luck, when you document the relative values of the aforementioned cars, you will find a repeatable pattern of the recreations pulling a consistent percentage of what the “real” cars bring.
Good luck!
TOM BRESKE
www.myvintageviewpoint.com
Seattle, WA