Thermal or Viscous? Clutch fan.

I got rid of a fixed fan, got an old Mopar fan clutch from a friend. Cleaned it up, installed it and it started leaking from the rear. Replaced it with a Hayden # 2947. The Hayden is 1/4" longer. Needed a radiator anyway, so went back to a cast iron pump that is 3/4" shorter than the aluminum pump.
I thought the 2947 Hayden was the shorter style??