The cluster in my 90 model has a curve in it and with the 87 tilt column swap in order to make everything look right I need the cluster to be straight across the bottom
I've seen a lot of people selling just the 90-93 140mph gauge for about $150. At least that's the regular going rate I see a lot but it varies. I recently just purchased an entire cluster for my 93 for only $120 and used it for parts to fix my speedo. I believe the 140 gauge should just pop right in to replace the 85 in the cluster, however I believe there are some calibration issues which I do not know off hand how to resolve. If you want to use the older cluster you'd just need the older style gauge. Mine was a 93 so it has the curve. If you are getting rid of your old cluster you could sell it to make some of your money back from switching clusters. It's also fairly easy to roll the odometer back on those if you want the new one to match the old gauge.
One thing to keep in mind, if you buy a used cluster it may have issues so if you can find a "new" boxed part and can afford it I'd highly recommend it.
#1 is to buy a 1989 Cluster which is a direct drop in. They switched to 140MPH halfway through the year, so only some have it. That makes it a very rare cluster so it tends to be pricey.
90-93 clusters need to be rewired to work, so leave this as last option.
You can also purchase an 89-93 140MPH speedo alone, and install it in your cluster. You just remove the needle, unscrew the mechanisn from the clear plastic support and swap it with your 85MPH speedo.
Then you just need a face. Go with an aftermarket overly, or try and retrofit the 90-93 140MPH face onto your older cluster.
When you reinstall the needle, take note to a small tick mark BELOW the 0MPH line. Remove the needle stop. With the gauge installed at proper orientation, the needle should point at this tick mark and NOT 0MPH. That way when you reinstall the needle stop, there's a little downward pressure on the stop.