harness has 4 other ground points and continuity checks out across the frame
Uh-uh ... 'fraid that's a classic newbie mistake; the later the Britbike, the fewer of any of the cycle parts are "ground" ... the Red wires poking out of the harness are (should be) connected together and to battery +ve - they
are the primary "ground", between a given Red wire terminal on an electrical component and battery +ve is the continuity you should
If the harness was for a '71 Triumph, the rear lamp might not have a Red wire (Lucas appears to have added it part-way through '71) and the turn signals won't but essentially they're the only components that didn't have either a discrete Red wire connection or one attached to their mounting (and they can be added easily).
So, if the engine has another Red wire connection from the harness, the one in your picture (attached to an oil tank mounting) was intended to attach to an electrical component's mounting - if it reaches, to the bracket where you've attached the turn signals relay? That bracket was originally where the rectifier mounted.
If I decide to connect the Ammeter - do I need to run leads directly from the battery for the most accurate reading?
It's quite a lot of wiring pain to connect an Ammeter to '71-on wiring, for not a lot of gain ...
For an Ammeter to work, the current must pass through it; standard Britbike Ammeters have a scale with a central zero and are connected between battery -ve, the original rectifier and the 'consumers' - ignition coils, bulbs, etc. via the ignition switch. In theory:-
. when the Ammeter needle's between zero and "-", the rider can see the 'consumers' are drawing from the battery;
. as engine and alternator rotor rpm increase, more is supplied by the alternator to the consumers, less is drawn from the battery so the Ammeter needle indicates closer and closer to zero;
. at a high-enough engine/alternator rpm, the alternator supplies all the 'consumers' in use and starts to charge the battery, then the Ammeter needle moves to between zero and "+" ...
However, in practice, the standard Ammeters were/are always small, low-quality and affected by vibration on a twin; apart from when the engine isn't running, at best, an owner gains experience of roughly where the Ammeter needle waggles with given consumers in use (essentially lights on or off) and makes an educated guess as to whether the battery's being charged, or not ...
When Ammeters were fitted to most bikes:-
. the Brown/Blue wire essentially went just from battery -ve to one Ammeter terminal;
. the other Ammeter terminal, original rectifier and Zener diode were connected by Brown/White wires to one ignition switch terminal; if you've replaced rectifier and Zener with a combined regulator/rectifier, its DC -ve wire is connected to Ammeter and ignition switch in place of the rectifier connection ...
... see what I mean about "quite a lot of wiring pain ... for not a lot of gain"?
Some will advise fitting a Voltmeter in place of the Ammeter - its wiring connections are just two wires, one to each battery terminal; 2" and 50 mm. OD Voltmeters can be had relatively-easily; even digital to avoid "waggly needle", albeit you need one with emi protection or the HT sends it doolally ... Ime and mho, the modern colour-changing/flashing LED do a better job of charging failure warning while costing less and taking up less space ...