Wiring in a volts gauge.

Yup. Look at the simplified diagram posted. The "welded splice" they are showing is a factory welded splice in the black ammeter wire, and it supplies several things depending on the year of the car. At the very least it supplies

the hot feed to the fuse box

main hot feed to the ignition switch

headlight (only) power to the headlight switch. So that original black ammeter wire MUST be powered.

Unless you are rewiring the car with an aftermarket harness, you need to hook the two together and taped under the dash, leave the black hooked to the alternator and leave the red hooked to the battery feed, as WELL as either bypassing or improving the connections through the bulkhead.

Then run an additional wire from the alternator to the battery. What this creates then, is the original wires become a PARALLEL battery feed INTO the car. In other words, electrically, the black ammeter wire is now actually hooked to the battery via the new bypass wire. This means that you have a larger gauge wire for a battery feed into the car