The Stovebolt Site Forums The Shop Area The Electrical Bay Help!! 12V conv. - Removing alternator pulley bolt

Help!!!
I just got a 12v Alternator and the other various pieces to convert my 3100 from 6V generator.
I got a replacement 5/8 pulley for the new Delco alternator but can't budge the old one! The nut is incredibly tight.
I have the alternator clamped in the vice on the workbench, with an allen wrench with 18" pipe over it with the workbench keeping that in tension.
Then I have 24" piece of pipe over a wrench to apply additional leverage on the nut.
I thought about an pneumatic impact wrench, but how do you hold the shaft from turning?
Would it work with the impact wrench on the shaft with an allen socket and holding the nut with the wrench wedges against a hard-stop?
The shaft allen face is beginning to round and I'm using quality tools - starting to get concerned it will strip beyond point of holding at all.
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'm usually pretty creative with things like this but this one giving me bother, especially as it's starting to strip the socket-cap.
Any suggestions before I do more damage to the allen socket-cap in the shaft would be greatly appreciated!!!!!