Keep trace diagram and simplified how I should connect '77 Voltmeter replacing with '75 ammeter.

1. Connect Black and Black line on Voltmeter. Both goes to Ground.

2. Add bullet male connector for Green/White line on Voltmeter.

3. Extend White/Red line to fuse box and connect to IGN after fuse.

 

I might just connect W/R, W, and G together to fuse box that connect to IGN line.

Nice work Tamo. Clear steps in the drawings.

Just a couple of questions. Where do you plan to connect the "10a fuse block" to? If the car doesn't have AC, you could use that.

The fuse cover notes a 10a fuse for the fuel gauge?

 

10A fuse should connect to other side of IGN.

I connect to fuel gauge/calibar/door.