In my opinion those Stainless flex o lite fans are hopeless. Heres why, when they try to pull the air through, the pitch of the fan blade flattens out,so they cannot pull through the volume of air required for cooling. You have basicly tracked the problem to airflow by your process of elimination.
Get a multibade stock fan with the thermostatic clutch hub and mount that up. Should solve your problem. You actually pick up horsepower and save gas on these fans. Because they only work in slow or stopped traffic. Because radiator temp goes up this causes the spring in the clutch hub to expand and engages the fan to the driveshaft, this pulls the air through when you need it most or rather when your engine needs it most. When you are in high speed criuse the airflow through the radiator cools the fluid and as a consequence the spring in the fan clutch contracts and disengages the fan which "idles" and you save a few horsepower that drives the fan.