I would like to chime in and offer some advice from a design angle whereas, the perverbial 502 will be a real challange to cool in that hot Texas heat.
First, let me say and you probably know that the 502 is a MARINE block which means the engine is really designed for high temperature gradient flashes, which means more iron in the jackets and smaller water jacket passages too (the weight difference between a 454 and 502 is 40 lbs...most of which is iron which inhibits cooling!). On top of the you are also dealing with the dreaded siamised cylinder design which allows no coolant between the cylinder walls (much like a 400 cube does). What am I telling you all this.....to get you to think about designing a cooling system that will handle this heat? If you start out with the largest airmover that you can get....you might get by with a heavy duty copper radiator, but an aluminum radiator upgrade is HEAVILY advised here.
The fan could be a Ford Mark VIII fan ....info here http://www.geocities.com/smithmonte/...rkVIII_Fan.htm
and I would drop the mechanical fan idea from the get go. I would NOT trust air pulling duties to a clutched system of any sort.
I have designed over 30 or so cooling systems on 502s this year alone all over the continent and I will tell you from experience alone that unless you have a beheamoth radiator out front, the electric fan will be the only way to keep your cool.
Just a friendly suggestion [img]graemlins/waving.gif[/img]