Well, I worked on my conversion from my original AMP gauge to an aftermarket VOLT gauge. I am posting pictures of the gauge installed into the original housing <-that I had to modify.
I removed the "aftermarket gauge Face" and drilled the "original gauge Face" so that I could use the original screws that held on the aftermarket face to the aftermarket volt gauge.
Tested after I installed and works great!... It took me about 10 hours to do this conversion.