> Now I can figure my pulley sizes to get the speeds I want. {found} a decent motor
If you have a likely-looking motor: mount it, cobble-up any likely size pulleys, and tie a transistor radio to the slow pulley. You actually don't want an RPM, you want a Leslie-wah, and when you spin the tranny radio you will instantly know too-fast/slow and know what size pulley to try next.
I suggest you start with the largest slow-pulley that you can fit. At low speeds like this, big pulleys always work better. Better belt-grip, more tolerant of belt-stretch, a less-tight bend over the small pulley. Your upper limit is the width of the horns (since they barely clear the cabinet), and I would not go smaller than half of that. Then I might try a "small pulley" that was just the motor-shaft: that's probably too slow but sure is cheap, and you'll quickly estimate how much bigger it needs to go to approach Leslie-speed.
(In high-speed drives, a belt on a big pulley moves so fast it lifts off the pulley. This is a real problem in engine cam-chains and a small problem in engine fan-belts, but I don't think a Leslie has either the speed or the space to have that problem.)
Actually-- this is another musical problem, only in beats not pitch. Listen to the test-rig while you tap a Leslie-beat. If you tap 7 times while the test-rig spins twice, then you need to spin 7/2 or 3.5 times faster and need a pulley 3.5 times bigger. If you used the 0.25" motor shaft, you need a 0.25"*3.5= 0.875" (7/8") pulley.
Though, for pulleys not a LOT bigger than the belt thickness, you should add the belt thickness (count half the thickness on each side; assume the middle of the belt moves at average belt speed). 0.25" shaft with a 0.1" thick belt acts like a 0.35" shaft with infinitely thin belt. So in this case you need 0.35"*3.5= 1.225" effective pulley, minus the belt thickness is a 1.025" real pulley. This isn't normally a big deal unless you start with a too-small pulley. (0.1" belt is really too thick to run happy on a 0.25" pulley.)
If you use V-belts, measure the pulley at the midline of the belt, not the rim or valley. V-belts run fine on large flat pulleys, you only need the V-groove on small pulleys.