mach1stang, about the best advice I could give your bud would be the advice the guys have me when I asked a similar question. Simply calculate the KV of the motor by the voltage you'll be supplying and shoot for about 35K rpm out of the motor, that said, 12 cells: 1.2v per cell * 12 = 14.4v so I would guess you'll be looking at something like an 8XL, maybe the 7Xl if your looking for blinding speed, but be VERY Carefull of heat on the 7 as it has a tendancy to run very hot.
That said, either motor would benefit greatly from Mikes Motor Clamp Heat Sync, almost a necessity on the XLs
As for speed, use BrianG speed calc to figure out your spur pinion combo to get you where you need to be speed wise.
Best O'Luck!