Keep it a Tempest, it will save you thousands on the restoration and making it a clone will not increase the value. You will see hundreds of GTO's on the road before you see one nice Tempest because they have all become parts cars or clones.
That does not mean you can't make it Go like a GTO. Heres mine all original body and 99% of the trim, i did swap to the GTO grilles and hood but have the originals restored and stored in the barn. The 326 is sitting on a stand along with the 2 speed trans untouched. It now has a 400 bored and stroked to 462. Being a high desert car the interior was baked when i got it so that was replaced with buckets and console in black. Color is match to the original Barrier blue with a very fine metallic silver flake. The link below the picture has around 500 photos throughout the restoration.