Holy shizzle! $30 YOU STOLE IT
Some simple disassembly - from the front side, not the rear - and you can swap your odometers between the two clusters. Pull the lens/bezel assembly from the front; no need to pull the metal back cover.
I haven't taken apart the 1991 needle cluster but I've taken apart many moonie clusters and then many 1992-up needle clusters. Most likely the 1991 needle is similar in construction to the 1992-up and you may need to gently pry around the edges of the gauge face to lift the gauges out of their sockets and access the odometer. Be careful with it as the gauge face is clear plexi with a sticker on it that is the black background and numbers. Don't break it.
If you choose to also calibrate the speedometer in your new cluster, read here:
http://www.chevytalk.org/fusionbb/showtopic.php?tid/258562
Richard