I'm pretty sure the way the parking brake mounts would have prevented that anyhow (see the next picture in the ad). Besides that, anytime you put a cabinet behind the driver's seat, you give up that function anyhow.
As it stands, I have a cable shifter for my 4x4 conversion in a similar place on my van and I've never missed being able to swivel the Driver's seat. (Passenger swivel is KEY, though.)