Aerohead's car reminds me a lot of Dick Kincheloe's Raptor, the 2nd pic in the original post. Here's the Raptor with some more of the body done:
I'm thinking tube-frame chassis, 1.5". The way Aerohead has his VW beam looks like it's almost under the driver's knees, which would keep the weight over the wheels. Just a matter of connecting the steering at that point.
Another idea was getting a Warrior type sandrail frame
and moving a couple of the rear crossbars around to secure a motorcycle. Considering the Warrior has a 95" wheelbase, you're looking at about 7' in front of the bike, so ~11' overall. Bonus, the Warrior is $399. I was thinking cutting foam down to squeeze between the rails, fiberglass over it all. That should keep the weight down, right?
Perhaps a 250cc bike?