I was typing when you posted. HOLY COW!!! I was thinking Chevy. It's always good to say you have a GMC!!! Did not see pic. I had a 56 GMC. Very easy to find gauge to fit. No need for expensive custom.$$$$$$$$$$$
They sell a gauge face kit, you can possibly re-face a new gauge with correct SWEEP. Use the amp face on a volt gauge.
The GMC gauges are standard diameter. They can be taken apart.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn