I'm with Steve on this one.
Every EB from 66 to 77 uses basically the same layout. The alternator's Black w/yellow output wire goes through the firewall and into the cab, is tapped for the fuse block, ignition switch and headlight switch, and continues on through the ammeter's induction loop. Near that point, there is a simple bullet connector and the wire color changes from Black w/yellow to either just Black, or Black w/red (depending on the year), and then back on out through the firewall to the battery.
So like said, EVERY AMP that flows from the alternator to the battery goes through that wire. And with the accessories tapped into the wire before the ammeter, when you show a positive charge, every bit of that indicated charge is read AFTER all the accessories get their feed. Which means that if your gauge reads 40 amps, you have 40 amps flowing from the point of the splice to the battery, but there might be more current on the alternator side of the splice.
It would be the reading on the gauge, PLUS whatever the accessories are using at the time.
The 78/79 ammeters are also shunted, just like the later model ones.
And they're notorious for never working.
An acquaintance used a 130 amp 3G with his Centech harness set up just like stock, with the Yellow wire attached to the alternator, the Red wire hooked to the battery, and no large cable hooked from the alternator to the battery. And while he did use it for a year with no obvious ill effects, he did change out to a larger directly connected charge wire anyway.
Just in case.
A friend with a '69 was not so lucky however. His setup with a 100+ amp GM alternator and dual batteries had a nuclear meltdown right in his driveway.
Something caused a high charge condition, and at least 2 fusible links didn't do their job, or had been removed at some point. I never did find any fusible links, but it was hard to tell what some parts of the harness were supposed to look like. Every inch of the harness looked like one big fusible link after a blowout! Melted plastic and some powdery blue copper strands everywhere.
Burned his whole harness right down to the bone in just a few seconds. I guess "luckily" though, that shut the engine down and ceased further carnage.
Fortunately, the guy with the Centech didn't have the same experience.
Paul