Guys,
I went to a local machinist, Tom Wiltse, who has bailed me out on previous occasions. My request was to weld a spacer on the inside of the pulley. He wasn't keen about the idea and proposed machining the pulley sheaves from the inside for half it's width, then machining a two step spacer to the right length and press it into place.
Worked like a charm. The change isn't even visible, and the pulley alignment is spot on. Now to move onto the next challenge.
Peter