Just to finish this thread, I decided to go with the elliot tacho rather than mess around with the old GTD unit.
I ordered direct from the manufacturer & they were a pleasure to deal with. I would happily reccomend them, great service, great price & a really nice tach, details at
Elliot Design
the unit fitted right in the dash, no problem & is wired straight to the MSD 6al, thus not needing the tach converter.