I have a 1974 Interstate Mark IIA that originally came with the European handlebars. Like you I had to do something about my back pain, and I had been told that rear sets would help out alot. So, I got a set of rear sets, and for me all it did was shift my upper body forward so that I was forced to hold myself up. Unfortunately my back pain did not go away. As I was riding, I spent more time trying to figure out ways to relieve the pain in my lower back. It turns out that my lower back was the problem, not the bike and I hope that is not your case. I recently had back surgery in hopes that down the road that my back will cease to be a problem.
I decided to go back to the original foot peg arrangement and I just installed the American bent handlebars. It looks as though the original brake master cylinder was made for the US bent handlebar. The hose exits out the end of the master cylinder and runs along the handlebar quite nicely. In contrast to the European bar, the hose and brake switch cover get smashed in the handlebar preventing you the ability to adjust the control lever to a position that I happen to like. To me the difference was amazing and you may want to consider that as well.
The Norton seat has absolutely no support for the lower back. I have a 1970 BSA 650 and the seat has a small rise in the middle of the seat. It is not that obvious to look at, but it really makes a difference in supporting the back. So I have been toying with the idea of modifying the Norton seat so it has something similar to my old BSA.