Tim, as Steptoe attested, it's a very time consuming procedure.
I did the swap on mine through neccessity some time ago but it took about a week to get it sorted. (I had a full head of hair when I started, too!!)
Just out of curiousity, can you get some photos of the connector blocks where the instrument wiring plugs into the main harness?
As yours is an 80G, it should have originally had the square guages on it, and it would show if either your main harness has been changed, and the guages swapped to suit, or there's been a mini adapter loom added, or the wiring has been murdered using a shi*load of bullet connectors to lace it all together.