Just a follow up on this.
a temperature switch - i.e. for an electric fan will fit this hole perfectly
you can then hide the wires along the firewall - making for a nice tidy install
The capillary gauge is more "period", trouble is the capillary is fragile and there is usually enough on the gauge to go 3 times around the engine bay.
An electric gauge with a sensor is easier but you do need a voltage stabiliser. I'm still working on the last bit.
i agree with the capillary length thing, bit of a pain, but that said...dad spent many hours trying to sort out electric gauge, and just couldn't get it to work! rather odd! and yes, he bought a stabiliser, and then bought a new type from maplins too...all rather odd! so he went capillary too!
i did break one of mine though...that was an expensive mishap!!