As noted by Joules, this is chassis 9628. I owned this with a "partner" in the UK and eventually sold it. Both transactions were what you'd euphemistically call "learning experiences", but that's a longer story!
FYI, note that this "race" chassis actually added only a few extra bars for support at the rear (e.g. the down-bars from the main hoop). It also had some pieces cut away from the stock chassis to make the engine/gearbox fit. So that's a good testament to the fact that the layout was fundamentally sound to building a stiff platform. Your layout looks just as good.
Glad this was helpful. I have many other pics of the chassis including those from the 1997 logbook, and as I recall there are more details about the Si chassis in various PI magazine back-issues.
Look forward to seeing more pics of your project as you progress!