Well AL I found something interesting. I remembered I had the front wiring harness out of a Nov of 65 built 66 chevelle with AC from the factory, except it did not have the gauge package. It still have every original connector and the non-sticky tap, grommets, etc. One Red wire (thinking it is 10R) from junction box to the splice for the horn relay, regulator, alternator, and power to fuse panel. And one Red wire (thinking it is 10R) from the horn relay to the splice leading to the horn relay. And one wire (thinking it is 12R from the splice to the alternator Just like what is in my car, except my car looks like the Red wire from the splice to the alternator looks to have been replaced with the same size wire 10R as coming from the junction box to the splice. And I have the wires for the ammeter gauge and this harness does not have those wires.
Maybe the wiring changed sometime before or after when this car or my car was built and the printing of the diagram. My wiring looks suspiciously like the 67 chevelle wiring less a fusible link and a fuse down the line from the split of the 10R from the junction box.
One of your questions earlier was about the shunt. You won't find a physical device in your system that is a shunt. Your shunt is a circuit formed by the way the 10 GA wire that crosses the radiator between the 2 amp gauge wires connected at each end of that line. The reason your gauge shows a high charge with the fan on high is because you are pulling power past this shunt since your a/c is pulling off the battery terminal.
If your harness has the 10 gauge wire currently from the junction block to the horn relay, you're good to go with a 61 amp alt. The 10 ga charge line from the alt to 4way connector is fine. Moving your take-off point for the under-dash a/c relay to the horn relay would get your amp gauge functioning properly. If the only wire you have between the junction block is a 12ga, then you are underwired and at risk.
I did receive your a/c diagrams and you said you hooked it up per the 65 diagram which has a relay. It shows hooking into the starter solenoid and powering through the battery cable. This is not ok because the power to charge the battery and run the a/c will need to flow through the small wire between the batt and rad terminal. If you run that ac supply wire to the rad terminal, then you would be ok, but still have the high reading on your amp gauge.
I can only speculate on the following...The wire between the batt and rad term is intentionally left at a smaller gauge. It's the wire that will smoke and burn should you have a short in the wires to the horn relay. Since it's such a small ga wire, I would replace it with a 10 ga wire to equal the gauge of the wire to your horn relay. I would install a 14ga fusible link in that wire. This main 14 ga fuse link protects the alternator charging line, the wires across the radiator support and the power bussbar at the horn relay.
Hooking up your ac per the 66 instruction would open another chapter of grief. It pulls all power off the ignition circuit which is already tasked with headlight, ignition, and just about every other circuit in the cab. The only good news is your amp gauge would read correctly.
More, by hooking the ac to the 4way connector or horn relay bussbar, your alt sense wire reads from this point and will work to give you proper voltage to the a/c and rest of the system just like the factory had in mind.