Deano,
I suppose you've read the MSD manual. If not you can see it here:
http://www.msdignition.com/pdf/6series.pdf
I have no experience with Smiths gauges, but I have a lot of faith in MSD products. They offer two adapters which are mentioned in the manual. One for magnetic pickups and another for white wire connections. Thats the way I'd go before sending the gauge off somewhere.
In any case, I wish you luck,
Paul