So, I did post this up elsewhere, but figured I'd put it here too to keep the whole build in one place.
I didn't do a play by play with pics, but I added the factory tach and small fuel guage into my 1990 cluster. On my cluster I had to dremeal out a 3 way partition for idiot lights for the fuel guage area. One was a batt (low voltage) light), one was the indicator for DRL's (hated that one anyways and the third one "low coolant". I have voltage and coolant and oil pressure guages, this was all redundant to me.
The small fuel guage shares a mount point with the temp guage. For the wiring I traced the old fuel guage wiring on the printed circuit and determined which wires did what and which pole they needed to be plugged into. I clipped those from the harness plug in and ran small female bullet connectors to the guage itself. So basically it is stand-alone like the tach.
For the tach there is a 3 wire harness. The mount points are all there, I just trimmed slightly in a few places so it would sit as intended, then screwed it down. There is already a square blank in the back of the cluster that needs to be chewed out to allow the three wire plug from the back of the tach to poke through.
I shot a little vid of it all working in the truck.
Volt guage is 1978
Oil pressure is 1978
Fuel is 1978
Temp is 1990, and the only 100% metric guage
Speedo is 1990 VSS
Tach is 1978
Cluster and Printed circuit is 1990
Rene