I was wondering about the viscous fan setup because I remember I had cooling problems with my warmed over 273, & I'm planning to use a 360 this time. I had the stock radiator, which is the same size as a slant six radiator as far as I can tell. The stock fan wouldn't cool the engine. I tried a fiberglass flex fan with the small, pointed blades & that couldn't do it either. I finally used one of the fans with the big stainless steel blades that flex at higher speed. I plan on modifying the core support opening so I can run a 22" radiator this time around. Was thinking about the fan clutch setup from Mopar (P4120758) because they say it can be worth up to 10 horsepower & 13 lb-ft of torque over a flex fan. They also say it can be 5 to 10 horsepower better than some other fan clutch/fan combos. For a little over $100 that sounds pretty cost effective. I'm just not sure there's enough room. I think my car used a 2" fan spacer. Anybody know if a fan clutch & fan is deeper than that? What about the fan diameter? I remember that mine came pretty close to the upper radiator hose.
Of course, there's always the electric fans. They probably give you just as much horsepower with only a slight increase in drag from the alternator.