If you take one of the original seats apart, you see why the hassle.
I have a picture somewhere of the bare frame, and it's really obvious how the seat back is offset over the center tunnel. It gets the headrest out of the way of the soft top frame.
The seat bottom actually goes through the frame and all but sits on the floor. The inboard rail of which ever seat you are in is almost directly underneath the corresponding butt cheek, while the other side the frame is well outboard.
As far as I know the Spitfire seats, and perhaps the GT6 seats are the only ones made in such a fashion.
I can just imagine the engineers sitting there in 1961 going "How the H-E- double hockey stick are we going to fit anything to sit on in there?
Personally I think with other seats in there you wind up more mashed against the door than the tunnel.
I rebuilt mine. Most expensive part of the project.