Taking into account that some one should find the lightest carcass one can find to start a featherweight project, I presume that one of these babies is the ticket right off the bat and at least 50-80 Lbs lighter than a fastback fish.
I could be wrong, but the slippery fastback rear window with all the metal quarter and roof surrounding it compared to this Notchback's lean rear probably amounts for a few porky pounds.