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You have a couple options. You can break it open and move the numbers around if you're real careful. They're designed to prevent people doing exactly that, though.
You can sit there with a drill and spin the speedo input until it matches the old odometer reading. That could take hours or days, depending on how far off it is.
You can just leave it the way it is. This is the route I took when I swapped instrument clusters a few years ago.
The speedometer portion of the clusters is separate so you can put your old speedometer head in the new cluster. Just take the clusters apart and each section should come out separately. I'm assuming your old speedometer head was still good, not the reason you replaced the cluster. If not, it is possible to rearrange the numbers. Takes patience though.
I can't say for sure, I've never had one of those apart. It works for 73-79, 80-86, and 92-96. Plus 87-91 looks like it's in three pieces, so it looks like it would work.