Do you see the round thing inside the rear quarter, just below the window in the door alongside? That is the stop for the seat back. There is a luggage well between the rear wheels.
I went out and measured:
- 42" from B-pillar to B-pillar
- 9" from B-pillar to driver seat-back
- 25" from B-pillar to rear seat back
- 4" from seat frame to luggage floor
- 36" between wheelwells
- 16" deep luggage well
With Type III engine cooling, one could cut out that firewall. Even use a Type III rear deck and engine hatch. The BGW van is the most aerodynamic. It adds a 3ft triangle of space. If space is a problem, cut the Beetle body long (and cut slits in the fiberglass's edge to re-contour that) and have a longer body for incrementally more work.
Most importantly you'd have opportunity to steal this air vent detail from the Citroen Tubik concept:
