Anders
In general high-flow fans are a low-speed remedy, a larger radiator is a high-load remedy. A high load can be a very hot day, an uphill road, or high speeds.
I think if you can answer two questions it will help people understand your problem:
(1) Is the 408 engine a 351C or a 351W?
(2) Do the cooling problems occur during stop and go driving, during high speed driving, or both?
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