We tried to change over a '75 Z1-B and were unsuccessful with the stock parts that we had available. The problem was the rear disc brace mount on the swingarm. On a disc setup, the brace mounts to the top of the swingarm. The drum brake brace is bottom mounted, so we had to try the later model swingarm. We had a '77-''78 KZ1000 model swingarm to work with and while the wheels fit the same in both swingarms, it would not fit in the '75 Z1 900 frame, it was just a tad wider. We were not prepared to do any grinding and welding modifications. The next obvious issue is mounting the rear master cylinder, the frame bracket isn't going to be there for that either. I have always wondered if a '76 B (with discs) model swingarm would fit in a Z1 frame, that would fix half the problem. So to the best of my memory to put the rear in you would need to take your stock swingarm and remove the bottom brace bracket, weld one (I'm not sure if the two are the same, I'm thinking different) on top and weld the master cylinder bracket into the frame, have everything painted or powder coated and you'd be good to go. 

The good news is that a dual disc front setup is a direct bolt on replacement!

Michael