Jempson,
I have just done the stanza upgrade to my '84 1200 ute. I had a twin reservoir to start with but I went to a booster from a 120y.
The block on my firewall was different to yours, as mentioned earlier the rear line came straight from the back.
I was lucky that I could manipulate the rear line to reach the new master cylinder but I did have to get a new line for the front to go from the cylinder to the block on the firewall. I just went to a brake place with my old line, told them the lenght I wanted and they made one up with the right ends on it and I bent it up to suit where I wanted it to go.
If you do go for a booster, don't forget (like I did) to get the adaptor that goes into the inlet manifold to connect the vacuum hose from the booster. The strange thing is, the one from a 120y will not fit the 1200 inlet manifold (well it didn't on mine). I got one from a skyline, the stanza one will also fit but it has a few connections on it you will have to blank off, the skyline one doesn't.
If you are going for a booster, you will also have to make a bracket to mount it properly because there is a kink in the firewall that makes 2 of the mounting bolts too short!! You will also have to do some minor adjustments to the pushrod connector where it attaches to the pedal.
One other piece of advice, If you still have the standard 12" wheels, try to organise new wheels before the upgrade because the standard wheels will not turn with the Girlock brakes on the Stanza struts.
Sorry for a long post but I hope it helps.