The best place to wire it is to the same place the original alternator output wire was ran. Inside the harness is a soldered junction with four red wires connected together, it is located in the main harness by the left headlight. They are all 12 gauge wires and one of them comes from the battery over the radiator, one goes from the junction to the firewall connector for cab power, one comes from the external voltage regulator, and the fourth one is the output wire for the alternator. If you don't plan to upgrade this wire to a 10 or 8 gauge, you can just splice it to the red wire from the voltage regulator since you will not use it any longer.
Here is what the junction looks like in my 67 harness. If you have a gauge dash panel you will also have a black or black/white stripe wire also connected with the four red wires. It is for the battery gauge (ammeter), and there should be a solid black wire connected to the junction bolt on the right fender by the battery which is the other battery gauge wire. If you wire the one-wire alternator to this junction bolt like so many guys do, the alternator will charge but the battery gauge will not show any charge or discharge.
Here's a picture of the soldered junction.
