Let's step back a moment. Just what are you upgrading? In other words, what year car did you start with? Are you just upgrading amperage, or going from the old 69/ earlier regulator to the newer 70/ later?
VOLTAGE!! Most these old girls have problems with the circuit path to the resistor. This is battery, starter relay, fuse link, bulkhead connector, ammeter, ignition switch, back out the bulkhead.........to the resistor
VOLTAGE DROP in that path will cause the new (or old) system to OVER charge (over voltage) One way around it is to cut loose all underhood loads fed by the ignition run wire to the ignition/ regulator and use a relay in that circuit
So far as a branch point, yes, you can connect there at the resistor, but what about the OLD regulator wire? it connects to the same point.
IN FACT if you are going from the older 69/ early regulator to the 70/ later, you can use the OLD wiring and ADD one wire just tap into the ballast as proposed and run that down to the second field connector on the new alternator
