where are the wires broken? at the speedo sender (i.e. before the plug) or from the plug to the loom?
if on the sender side, i recommend you just replace the sender, you'll have to get one from a wrecker, i don't think you're limited to just u12s, don't forget there are ford corsairs as well but i believe a few other nissans of the same vintage have similar senders.
the problem with these is that you'll get oil and other crap on from the engine on the sender, right where the wires go into the unit gets quite weak and you can break them with the lightest touch. i imagine that because you've just done the clutch you've probably unwittingly broken the wires when you took the box off - i've been there done that.
sender shouldn't be more than $10 from a wrecker.