In my head I always think of gathering the air into the radiator with ducting, so whatever air is trapped by the ducting HAS TO go through the radiator. Then on the back side, provide a nice open area or vacuum to carry the hot air away. I just ordered my radiator today, 8" tall, 29" wide. I'm hoping that with good ducting and 3 8" fans pulling from behind that it'll be enough.
The proof is in the pudding, so really, ask this guy
Zenjoes Rotary Bug to see what worked for him and many like him using the rotary motor. There is also
VolksSwap, a related site.
Jason