In a message dated 9/20/99 10:03:14 PM Eastern Daylight Time,
msnichols@MINDSPRING.COM writes:

<< Good evening all.  I have acquired  the tach/fuel gauge/water temp gauge
 from an '89 for my '81. ( Yes, I know the water temp will not go, but it
 fills a hole in the tach faceplate, so I'll leave it.)  I could find
 nothing in the archives on this installation.
 ***Has anyone done this installation into the dash cluster on an '81??  I
 would rather keep my dash and modify the tach to work with the new fuel
 gauge.  I did get the printed circuitry with the new '89 fuel/temp/tach.
 Any help would be greatly appreciated!
  >>

this has been done plenty of times. there is a bunch of stuff in the archives
on it, and in the LiMBO newsletter.
basically, you have to switch the pins around in the multipoint pin
connector, and run a wire from your coil to #14 pin on your connector.
chris