Thanks for the replies so far, guys.

Cudajon, a belated thank you for taking the time to post your voltmeter conversion instructions. I was unaware that a top-pivot voltmeter existed until you clued me in. Once I saw that, the rest fell into place.

Anyhow, the B-Body ammeter face is rectangular and much wider in the horizontal dimension than the Sunpro gauge face. If you just used the Sunpro gauge faceplate, you'd see the back of the instrument cluster at 3 and 9 o'clock.

I have a spare cluster and tried laying the Sunpro gauge face over the Coronet's ammeter face. Imagine a circle on top of a rectangle. It was very noticeable.

I'll take a few pics tonight and post them.