Late to the thread, I know, but I also have an SJ21 Mk 1 and my interior looks just like yours. I believe those large bulkheads were only on the Mk 2s...and I know a handful of Mk 2 owners that have cut them back or replaced them with much thinner metal bulkheads in order to open up the cabin some more.
And yes, you can open up that box, it's hollow inside. Luckily the former owner of mine had taken care of that already.