Do you have a shroud? Because you may want to make sure a 18" fan will clear the shroud. I use a 67 shroud in my 68, and I had to go with 17" fan because an 18" incher didn't leave enough space around the circumference.
FWIW- I use a 17117 derale fan with a 2747 hd hayden clutch that hayden put a shorter jag-style 2765 shaft on. It cools great. Don't spend big bucks on a pre-assembled kit when you can buy them seperate for around $30-$40 each respectively (fan and clutch).
