Brian,
The reason I suggested putting a fan on the rear of the radiator to pull air through, is because I have done that on my 411.
I'd had air conditioning installed in the 411 and once the radiator had a condenser placed in front of it, the cooling system as no longer sufficient. The original A/C installers had bodged the job so as part of a much larger project the A/C installation was re-done and I was persuaded into replacing the radiator and reconfiguring the cooling system. We removed the original fans and cowling and installed the new radiator forward of the original position, with two modern electric fans sandwiched between the condenser and the radiator. Unfortunately this was still borderline in terms of cooling on a 30+ degree day, sat in traffic. So we added a large electric fan to the rear of the radiator; this dramatically improved the cooling, so even on the hottest days the engine will not overheat.
As far as I am aware there was no consideration given to the speed of the front and rear fans, but it definitely works.
We also did away with the original Otter switch for the electric fans, which I believe the 409 also has, and replaced it with a modern variable thermostat which can be set to turn the fans on a little earlier that the original 98 degrees C. We also fitted a separate ATF cooler.
If I was a 409 owner with a borderline cooling system, I'd probably start by just mounting a slim electric fan directly of the rear of the radiator and see what difference it makes. It's a relatively small cost in the scheme of things. You might need to install a relay to power the new fan, triggered from the original fan power feed.
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