Ammeter to Voltmeter...who does it?

Your 12V source SHOULD reflect the actual battery voltage. Others on here, as well as myself have discussed the problems with voltage drop in the harness. Even if you bypass the ammeter, the ignition switch / connector, and feed out through the "ignition run" circuit can still have drop, cause overcharging at the battery, and a wrong reading on your voltmeter.

One way around that is to use the original "ignition run" wire out in the engine bay to trigger a relay, then feed the alternator field/ regulator / ignition / etc off the relay. This in most cases will "work around" the remaining voltage drop problem, as well as relieve the under-dash circuit enough that your meter will read more closely

As far as the MAD article, it's as good as any with factory wiring. One thing you must understand, there, is that they were trying to do that mod with as little as possible into the appearance of factory. Part of the reason it was done that way is to provide additional wire capacity (ampacity) into the interior of the car