Well if it is the gearbox, like you said, it might be worth just sticking a lower milage engine and box in. But thats hassle and money!
How about you take the speedo out and just go for a little drive up the road or something and just see if the end of the cable operates from where it goes into the back of the speedo? That would be the best place to start, because it saves half hour or so fannying around taking out the cable, replacing it to find out it could be the speedo itself thats decided to go...
Id do that first, then if the cable doesnt work, you know its that, if it does work, stick your clocks from your si on there for a split second to see if it works... problemo sorted.
