Drawing from a Lucas manual showing the sizing refers to the overall diameter:

Roadster non-standard A127 alternator with the belt a couple of 3mm below the edge of the flange:

V8 original (AFAIK) Delco with the belt 5mm below the edge of the flange:

Metro alt with the belt almost flush with the edge of the flange:

Right- and left-handed alternators:

Wrap a belt round the pulley and holding that should be enough to keep the pulley and shaft still while an impact gun removes the nut:

No Woodruff key on this alt unlike many, just a friction grip. There is a hex socket in the end of the shaft which may be enough to hold it still while using a spanner on the nut:

By removing the three through-bolts (which may need removal of the pulley and fan) you can rotate the halves relative to each other until the rear mount ear is in line with the front adjuster ear, changing that to the front mount ear, and what was the front mount ear is now the adjuster ear. Shown half-way between the two positions:

The bush in the rear mount (arrowed) is a sliding friction fit and will be pushed into the correct position as the two mounting bolts are tightened in a trial-fit. Then as well as checking the output voltage with as much electrical stuff switched on as you can, check the alignment of the pulley and the water pump with a straight-edge. If it's too far back then you can move the alternator forwards with packing washes between the bracket and the front mount. If it's too far forwards then there may be spacers on the shaft that can be reduced in size.