They are just manifolds, and cobras have oval exhaust ports while you have circular, so they wouldnt work. Cobras make the power with the heads and the blower.
Im not sure that Id buy a car that was making 513 hp on a stock block, and if he tells you it did that on 8psi, he's lying. Have it thoroughly checked out before ANY exchange of cash.
:D You should drive down to the local dealer that wanted to mark up the GT and say, see, you couldve sold it to me, but now look what you did.
Enjoy the Z man, they are amazing cars.