Okay guys, just following up- the overheating issue seems to be solved.
The replacement heavy duty Hayden 2765 fan clutch seemed to help a little bit, but the temperature still eventually crept up. I added some Water Wetter as a last ditch effort and maybe got a 2-3 degree improvement, but again the car would eventually get hotter than I'd like at idle. I ended up ordering a 17" high output Derale electric fan and a SPAL thermal relay kit. SPAL includes both a 30 and 40 amp fuse in their kit along with eyelet connectors, a fuse holder, all the wiring, thermostatic switch, and a bunch of other bits and pieces. The fan pulls around 23 amps- I found that the initial power spike on fan startup was too high for the recommended (per Derale) 30 amp fuse, so a 40 amp fuse was used instead. I attribute this to ditching the low speed setting and wiring the fan so that it immediately goes to high speed. The fan kicks in at 195* and shuts off at 185*. Problem solved!
The fan is off almost all of the time and only comes on at idle when needed. It's significantly quieter than the mechanical fan and I don't have to worry about losing any fingers. One thing I did notice however is how difficult it is to get hot air OUT of the engine bay as it takes longer for the fans to bring the temperature down with the hood closed. I'll try and get some pictures of the setup later today.
I took the fastback on a rather arduous cruise with a few mustang buddies yesterday and she ran flawlessly!
IMG_1865 by
armon7, on Flickr
IMG_1897 by
armon7, on Flickr