There is a rubber gromet directly behind the instrument binacle covering a hole where the speedo cable goes through if I recal correctly. I fed my wires through there. Its easy to find from the engine bay.
Re the water temp sender I had an after market one on my old 24v. I cut the top radiator hose and used an adapter piece with sender housing (about £20 from Demon Tweaks) to join the two pieces back together.
Found a link here (make sure you get the right diameter and thread size):
http://www.demon-tweeks.co.uk/produc...&pcode=GLOHA25