@Rob - sorry for the delay, just did some further checking.
Unfortunately my spare B234 didn't have an alternator fitted, but it still has the bracket on the engine.
Since I still had a B30 alternator, I tried to see if things would fit the other way around. If it works one way, it ought to work the other way, right? Well... maybe.
The B30 alternator takes a larger bolt than the B234 alloy bracket is prepared to take. As you can see, the two mounting holes take fairly small bolts.
The B30 alternator's mounting bolt is significantly larger, although the spacing seems about right.
It appears that if you drilled out the mounting hole in a later model alternator, the pulleys would line up - or at least, if you placed a sleeve in the B30 alternator's mounting hole, it would allow the pulleys to line up on the B234 - see below picture.
But without a late-model alternator at hand, it's impossible to say if it has enough meat around the mounting to be okay with a bigger hole. So IMHO Angus' suggestion (swapping the dead B30 unit's front casing etc onto a Commodore unit) to be the most cost effective and reliable.