Here is a brief description of what the wires do on the connector plug of your Alternator.
You need to be aware of the two brown cables on the large terminals SENSE and OUTPUT
There were several different connection arrangements for Lucas alternators over the years ranging from 4-pin of the 16AC with remote regulator (best avoided for a conversion), then a 5-pin using two connectors on the early internally regulated 16ACR and finally a 3-pin single connector for other 16/17/18ACR variants.
5-pin/two plug systems have two Indicator spades in one of the connectors which are linked together by a loop of brown/yellow wire in the plug, possibly to protect the alternator if the engine is run with the IND/B+ plug removed. 3-pin have two variants - one with two large spades side-by-side and a single normal-sized spade to one side, and another with a single large spade and two standard-sized (or one standard and one medium) spades either side of it although this may be from only one supplier.
With first 3-pin type there seems to have been two variations of how the spades were used - on one the central large spade is the output and the other large spade is the battery sense terminal, with the normal-sized spade being the IND terminal, and on the other both large spades are outputs where either (or both together for more current carrying capacity) can be used, and the normal-sized spade is the Indicator terminal.
Where provided the B+ (or BATT+) is a battery voltage sensing terminal wired back to the solenoid with a standard gauge brown wire. This is used to sense the voltage at the solenoid rather than the alternator for voltage regulation purposes, and would ensure that under high current conditions any volt-drop occurring in the main output wires (thick brown and black) between alternator and solenoid/body is ignored and the voltage at the solenoid (and hence the battery) was maintained at the correct level. This was the case in the 5-pin 2-plug 16ACR from 69 to 71.
Initially the 3-pin single-plug alternators used machine sensing (i.e. the correct voltage was maintained at the alternator terminals, but could be lower at the solenoid and hence battery under high current conditions) with just a single thick brown and a standard gauge brown/yellow in the alternator plug.
Possibly because of problems with low battery voltage, in 1973 the alternators seem to have reverted to battery sensing again (Clausager states the 17ACR was fitted from February 73) now with an additional thin brown in the alternator plug wired back to the solenoid as before, and this seems to have remained the case up to and including the 77 model year. This is a 3-wire alternator, but can be used on a 2-wire circuit by connecting the third spade to the output spade in the alternator plug.
18ACR, This version seems to have gone back to machine sensing again, with two large gauge brown wires in the alternator plug, and the two large spades on the alternator are both output terminals. This gives increased current carrying capacity and lower volt-drop now cars had electric cooling fans, offsetting the loss in voltage caused by the regulator sense terminal moving from the solenoid to the alternator again. This is a '2-wire' alternator again.
So some care needs to be taken to determine just which type of wiring, plug and alternator you have when making changes, even swapping alternators which take the same plug. If by looking at the two large spades on the alternator you can see they are clearly connected together, then you have a machine-sensing alternator and can use either or both large spades for the output. But if the two are clearly insulated from one another, then you have a battery sensing alternator. On these you must have a large gauge brown wire on the output spade at the very least, and a smaller gauge at least on the sense terminal. If in doubt as to which you have, it may be possible to determine by voltage measurement.
Turn all the electrical loads on you possibly can, alternator plugged in, engine running at a fast idle, then connect a voltmeter between the two large spades. If you can measure any voltage between the two (may only be in the order of tenths of a volt) then you probably have a battery sensing alternator. If there is zero volts between the two large spades, then you probably have a machine sensing alternator. Or simply provide large gauge brown wires to both large spades to cover both eventualities, and get the benefit of a lower volt-drop under high-current conditions if you have a machine sensing alternator.