I have rebuilt one and it isn’t terribly difficult. I also used a reproduction decal kit and it turned out well. The worst part is taking off the speedometer needle off its pivot point. That is the absolute worst part because of the VERY fragile nature of that needle on the shaft. After that, go slowly, take pictures, and you’ll have it back together in no time.
Here is a sort of “before and after” of two speedos I had. Both were in this kind of rough shape when I started. You can do it. This was the very first speedo I had rebuilt and it went fine. Just be CAREFUL taking off the needle. These old chevs\GMCs aren’t sitting around in farmers’ fields in the Netherlands like they are in Canada and the US! I had 3 speedometers as backups just to be sure I had enough parts, in case I broke something.https://photos.app.goo.gl/YEOFa2AbSgJQURDt2