Have you considered using a V8 supercar front bar/spoiler?
It could easily form the start for a belly pan.
I've spend a lot of time comparing VT/VX to VY/VZ commodores when I see them at the lights or on the free way. Those rounded edges on the back of the VT look better but are probably creating more turbulence.
Also interesting (and I see this on a lot of later model cars) is the 'fake diffuser'
I''d love to get one and open it up and make a proper diffuser and see the difference.