If memory serves, the theory is if the fan blades are too far into the shroud, they create turbulence inside the shroud and don't really pull air like they should. Too far out and of course the fan blades can start to suck air from outside the shroud. Yours sounds like it's pretty close so I doubt that's the problem. Have you tried a different fan clutch?
Daytona: Don't have a clutch fan, no room. Have the factory fixed 7 blade 18 inch #2863224 fan. Car is a big block 69 A-body so it only has the factory 22 inch radiator like they came with back then. But the rad was recored to 3 core.
