Has any one out there tried fitting a golf (MK3) master cylinder? I went to fit the sierra one and some idiot has welded a diagonal brace in the way, I could cut it out and move it but that means refitting the engine etc. The golf cylinder is nice and short and will fit but i'm not sure of the bore size etc.

Are there any markings on the side of the cylinder that may give an indication to ID?
If you get stuck, go to your local brake specialist, or even a friendly garage, and look up the specs in a catalogue. Some catalogues even have a cross refference section.