The info poublished at GMCPauls website is very, very helpful. I just finished a swap from a 3-hole I-Light cluster to a 7 hole (standard full gauge) cluster.
There was a little tedium to the process and I made two mistkes that made it more difficult for me, but joy was had in the end..
Note 1: Lable all of the wires before you push the pins out of the socket -- some wire colors vary from one harness to another and some colors are not all that different after 30+ years of age. Being able to identify where the wire was in the original location will help a bunch when you go to put them all back.
Note 2: You don't use the old I-Lite wire for the ammeter, you need to run two new wires as shown in the help chart.
Tips page at GMCPauls :
http://www.gmcpauls.com/Tech%20Tips1.htm