Another thought was to hold the pulley in the vice and just try to crack the nut with a socket and breaker bar; but looking at the new pulley, it's not keyed so I fear it would just slip.
I managed to achieve the result by this method.
The shaft indeed still turned (no surprise there) but now with the pulley clamped tight, as I tried to loosen the nut with the socket & breaker, what happened is the lock washer opened up because the nut was now turning against that.
So after it relieved the preliminary torque was pretty simple to get off.
I think my fundamental mistake was that I was using 'inch' tools - although the allen wrench had
seemed like a good tight fit, I was able to use a metric one to tighten it up & got really good torque on it without it rounding.
I guess I just assumed American alternator (plus setting out to work on the '51!!!) it would be std. I'm not sure however I still would have been able to crack it - I had tremendous torque on it before it started to round.
For anyone else in this situation, the only caveat is that the pulley & washer are pretty worthless afterward - that wasn't a concern for me with the new pulley going on anyway and frankly no room for washer with the new pulley (although it would be to source another if required). I just applied Loctite for extra security & torqued it really good with my extender pipes.
Incidentally it may seem like I'm talking to myself, but I know the forum gave me inspiration!!! Made me think as I was typing the request too!
