The average length of the fuel cell portion of the tub isn't that great, and the seat bulkhead is substantial and bonded in. I would think that part of the tub is pretty stiff even without the rear bulkhead bolted on.
The cross section mainly governs - in general terms - rather than the longitudinal length: its shape, closed is much much more stiffer than open, in particular in torsion [the cockpit opening is usually the weak area for this reason]; the area enclosed by the cross section, in torsion; the depth, in bending.
The aft bulkhead itself usually doesn't take directly the loads the engine attach points is feeding. If so, as it can happen in prototypes with wider chassis, then the bulkhead plate must be adequately stiffened to take those out-of-plane
The engine attachment points are usually located on the contour of the outer shell, as the picture posted clearly shows: in this case, two in the bottom plate to the crankcase and two pairs either side to the cylinder heads. The reason of course is that shells work better structurally when loads lie in their plane.
The bulkhead is obviously required to stiffen – or better, to stabilize – the cross section contour and allow the spreading of the loads in the outer shell of the chassis, in particular into the two composite materials faces separated by the honeycomb core.
All this pathetic lecture, just to say that a picture of the bulkhead bolted on would be nice to see.
Any time, no rush. Thank you.