I would probably recommend seam welding around where the inner wings and turrets meet, and at the large folded seam under the rear side windows. Also there is a large stiffening bracket that joins the vertical inner panel to the horizontal box section that the rear seat belts attach too. Seaming this is probably a good idea too. As the ARB is within the rear beam, ARB loads go into the 2 beam mounts (these a pretty well attached as std with big spot welds, but again a seam weld might be belt and braces here!), however the damper loads are fed straight into the inner wings, and this loading is perpendicular to the roll cage cross brace. TBH on a front engined FWD car, there is typically so little weight in the back, and you tend to run softish damping you shouldn't get a big
problem.
The main reason to ensure that the outer wings are braced to the inners is in the case of a side impact, Ok in a biggie then the car will be a right off, but if you just clip something, having the outer wing supported may prevent damage to the suspension / wheel / tyre behind it.
Like you say they are identical, think mines deffinately had springs from new, as it sits alot lower than yours though.
