Do a liger..I'd think the bolt pattern would be closer (total guess) and you'll get more displacement. I believe the cr500 and fl400 strokes are both 79mm so the liger would be cast close for the stock stroke.
I've seen the pilot cylinders on a 250r bottom end, what about a Saber on a Pilot bottom end? Just picked up my second pilot this evening, this one is going to be mine... mostly... hehe....