John. Well done, you know it makes sense.
Your project seems to have many advantages over my starting point and the more original parts that you have, the easier it will be.
I, to, are of the opinion that the rear light extension was only fitted to the 'west coast' model to comply with the regulation that the rear light must be at the very back of the bike. All of the European models that I have seen pictures of (Quite a few were sold in Europe towards the end, when they were getting harder to sell in the US) do not have this part fitted, (in fact 5 times as many Super Flashes were sold in France as Rocket Gold Stars!!!!)
Anyway, its good to see another S/F reside in the UK, we will soon qualify for our own 'sub section' Pete