Hi Ian,
that looks like the original John Hurst (DGT / Deon) chassis to me. My guess would be that unless custom axles were specced it would have the same rear track as the front track of the donor Lancia Beta. One site I googled put that at 1410mm front Beta Coupe track - pretty close to a genuine Dino.
As for removing the central tunnel - my uneducated guess would be that you'd have to compensate with other high beams or beef up the beam's section at low level. I've been thinking of three longitudinal 25x25mm rails. Two as high and wide as possible under the top of the sill and one as low as possible. short upright tubes join all three with triangulation in all planes.
206_sill_idea_Rossnzwpi.jpg
206_sill_idea-2_Rossnzwpi.jpg
and here's a genuine 206GT - see how broad and high the sills are
206_sill_idea-3_Rossnzwpi.jpg
cheers
Ross (in NZ)