Time for an update.
My original cluster had a broken speedo. It broke almost immediately after I bought the truck while leaving LA.
My plan was to do a custom setup to display RPM and oil pressure
on my new instrument cluster (my original cluster did not have tach or oil pressure gauge). Everything on the new cluster worked (including speedo), but for a variety of reasons, I decided to go back to the original cluster, rather than fool with RPM and oil pressure.
But I still needed a speedo. My friend Matt reminded me that I could probably just swap the speedo out of the new cluster into the old cluster, and then I could reinstall my original cluster (with working speedometer).
Original (broken) speedo on the left. New working speedo on the right.
Voilà. Working speedo in original cluster.
The script for the timing belt reminder is blocked, but otherwise everything works great. Well...except for that sometime while the truck was sitting, the fuel gauge quit working (unrelated to any cluster swap shenanigans). I have ordered a new fuel gauge float and hopefully that solves everything 100%.
Huge thanks to
@FJ73Texas for all his help. I could not have done it without you!