The bottom pic is the clearest exposition of your concept. By my onscreen measurement, the wheelbase discrepancy is 94%. The discrepancy in wheel diameter is 88%. Imagine a mannequin into the driver's seat. The windshield header will be coincident with the driver's forehead. But enough about the details.
I will be making a virtual build using 3d models including more compact seats to make sure theres proper spacing .
There are things that are hard to move on a car body, like the cowl height and beltline. That's why you see show cars that are sectioned or have a chopped top, but what you don't see as much are slab sides and beltlines resculpted as you show.
I feel cowl height may be a little problem once the seats are lower... but hopefully not too bad.
Try this: Start with the stock Previa body and apply a slant chop. The stock roofline's highest point is over the middle seats. Essentially you would cut a slit across the top of the windshield and take 6-8" out of the rear posts. The rear seat disappears and the middle seat moves down and back (else a shag carpet and water bed interior).
I think the Prius top would be perfect for this... I'd just have to adopt the Prius power windows to work with the Previa controls.
The roof could follow Thee Holy Template which would gain a lot of cred on Ecomodder. I would then graft in the rear hatch from a Dodge Magnum since I rilly dig those. The aggressive fender flares could be body-kitted over the slab sides.
Im thinking more of the Lambo SUV for the rear hatch... and FORD Bronco concept for the sides as I already have the 3D's for them.
You could start the slant chop at the bottom of the windshield, but good luck with that.
Edit: Oh, my. You could have started over fresh at Ecomodder, but you posted that VWVortex thread. They're not very polite to you there, are they?
We have a love hate relationship 
So the way I get it is that you are living in the Previa in question, with a CNC router taking up the whole back end. Is that about it?
No I live in an apartment 
but yes my CNC machine is in the back of the van