Hi Tinkertoy,
I can't resist throwing in my 2 cents, but Flux and Joe have you steered around the right way, I think.
I've done a few conversions, so far without re-winding any of them. Good power from each, though one didn't have enough magnet and one had crappy wiring. That said, I have to admit that I could improve them with re-winding, if I did so. The magnets get you so far, which may be enough, or plenty depending on your expectations. But if you truly want a perfect match between the prop's power curve and the generator's load curve, and match both of them to the wind available at a site, then a re-wind may be necessary. At your stage of the game, I don't think you need it. A motor conversion can always be re-wound after the magnet job, if it needs some extra "tweaking".
In my experience it's all about the GAP. The more flux passing through that gap, the better. The smaller the gap between magnet face and stator tooth the better. If you put a flat magnet on a round rotor surface, there's a gap there too. I've had best results from machining flats on the rotors when I convert them. I've seen other guys just turn them in a lathe and use a file to make the flat spots. Many people allow for some cut/fit/cut/fit as they trim down the rotor. But watch out for pinched fingers as you keep putting the rotor into the stator with all those magnets pulling it.
Smaller magnets allow for smaller gap (also fit on a round rotor with less gap, if you must), but it does increase the cost and time required to finish the job. I spent months on my Baldor conversion, partly because of the intricate machining (done in my spare time) and partly because I could only clamp a couple of magnets in place at t time, and there were 48 of the little bastards.
The motor you're starting with sounds similar to the GE and Baldor motors that I converted a few years ago (also 3ph / 3HP / dual-voltage). Can yours also be wired star/delta? Can you say how many wire leads are available? You should have at least 6 but you may have 9 or 12. Give you a lot of flexibility to fine-tune the speed range when the magnet rotor is done.