You will have to move around the wires in the connector on the harness in order for it to function since the fuel gauge is on the opposite side of the connector. Just look at the backside of the 2 clusters and make a chart of each of them listing the location of each terminal.
As you are looking at the back side of the cluster assembly the terminals are numbers on what is to YOUR RIGHT FROM TOP TO BOTTOM 1-9 and on what is to YOUR LEFT BOTTOM TO TOP 10-18.
Trace the leads on the printed circuits from each light & gauge terminal and make a note as to where they go on the original cluster. then do the same for the replacement cluster. Compare them and make the necessary changes to your wiring harness. To do this you will need to re-pin it by removing the terminals from the housing and repositioning them where you need to
NOTE: I WOULD STRONGLY SUGGEST MAKING A CHART OF THE HARNESS LISTING BOTH THE WIRES USAGE AS WELL AS IT'S COLORS BEFORE EMBARKING ON THIS
As far as hooking up the tach to work, there are 3 terminals on the backside that are marked 12V, GROUND & COIL
Hook the 12V (PINK) lead into the fuse block at any "IGN" source NOT a "BAT" source This way the tach is only powered up when the key is on. Run the "GROUND lead (BLACK) over to the 6-GANG ground terminal located on the left kick panel behind the dash. The last lead should be white or even brown it needs to be fed through the firewall and you can do that by bending an old wire hanger and shoving it up and through the same grommet your speedo cable goes through. to hook it up under the hood. there is a white wire located right near the coil with that plugs into the air. This is the tach signal lead from the coil mostly used for diagnostic testing.
Below is a pic of the chart i made for my suburban before the change over and of the back side of the tach connector so you can see what I'm talking about.
