That's your key.  It wants four poles (2 north, 2 south).

for a 4 pole 36 slot stator, the minimum span the coils can be is a span of slots 1 and 8, the maximum can be 1 and 10.  that was for a lap winding 2 cpoils per slot, 36 coils total.  for a concentric winding, it is most likely coil spans of 8-10-12, and likely what baldor uses, (18 coils total),  as long as it is a 4 pole winding (probably name plated 1725 rpm).

so for a lap winding of coil span 1-8, and a concentric winding of 8-10-12, the width of the mag should be the same as the width of 7 stator teeth. in either case, this will give you a mag that fits right between the coil legs.

use what ever mags you wish (i would suggest using 3/4 wide and 3/8 thick) to form the proper mag width and length. the length should be the same as the stator lamination thickness. you will also notice that the actual length of laminations in the rotor is the same as that of the stator. be sure to angle your magnets at 10 degrees to reduce any cogg to a minimum.  i find laying it out on paper and then taping the paper to the turned down rotor, and then centre punching the cross hairs on the paper makes a very accurate way to do it.

here is a link to the irc gallery that shows what i mean about laying your magnet arrangement out on paper.
http://www.anotherpower.com/gallery/zubbly?page=16

My first conversion started out with a 7.5 hp but I going to try something a little smaller. 400 dollars for magnets and rewinding might more the I can stomach. Making a 18 foot prop with no experience might be a receipe for disaster. Then there's the tower. Better to experiment with my 3 hp. Then I read about zubby's 25 hp! Good luck on that one.