I run 7-blade fans and hydrostatic clutches on several cars, including a 375 hp 406. No problems at all.
I don't recommend flex fans. They're noisy, they don't cool well and they have a habit of failing catastrophically....like throwing blades thru the hood.
I'd think that the failure you had was a fluke. An imbalanced fan or a faulty clutch. Btw, when the clutches go bad, they can lock up and run all the time. That's real noisy and could weaken the fan if run for a long period of time that way.
I really don't have any problems with the 7-blade and hydrostatic clutches. I put one on my 292 to replace the solid mount and it's workin fine.
