Zeta Impala made it to fibreglass, like the Coupe60 (which was the GTO/Monaro proto) and the Ciel, which I think was the short 'big Caddy' proto. Fibreglass is the last stage before metal - they do it because even though clay gives you a good idea of the final 'look' nothing is better than seeing a car going around a corner, accelerating, braking whatever to work out what it looks like as a roller.
The cost to create a car like this is $millions - not only in the clay modelling, which takes thousands of hours (I was told the cost approaches $10K an hour) but the model makers have to literally make the car complete in glass with all the details like bodywork flutes and apertures, rain gutters, moulding and glass openings. The Coupe60 which used a standard G8/Commodore floor/front panels cost $8m just to the stage where it was b-pillar back and doors, trunklid. No interior, no real production fittings - just the bodyshell.
Once they do that, they literally photograph it (old days for the press toolers) or laser measure it for the CNC tool cutters (today). IOW, the Zeta Impala was months from manufacture with serious production intent when they pulled the pin. Just imagine, that car powered by V6s with 6L45s, n/a V8s with 6L60 and Tremecs and supercharged motors with 6L80s........ with the IRS capability of the G8, SS and Caprice PPV, it would have ushered in a new era of RWD sedans hinted at by the Chrylser 300/Charger.
In Zeta II it would have had similar mass to the FWD Epsilon but much better 50:50 balance - no issues with width, overhangs, and MonaroSS's rendering from a few years back showed that the '14 Impala styling, that looked like a whale balanced on a pram with the narrow/short Epi II wheelbase looked much better with the wheels pushed back out into the corners and the rear doors/compartment a proper length.
Next time you see a Caprice PPV, see if you can sit in back (without a ride down to the hooscow!). You will be amazed at the spaciousness in the rear, it's like a cave - you could get Lebron and two of his mates and their groupies in there, with the fuel cells below your feet, and in the trunk where a 20x10" spare will fit underneath the floor leaving it unrestricted. It makes a Crown Vic (been in both) feel like a long sardine can and shames it for trunk space with the spare mounted.