Stefano,
Are you sure your problem is not a fuel supply volume and/or pressure issue, and/or perhaps the wrong carburettor?
I have a 450 cu inch engine in my 411 which I am sure puts out just as much heat as your 318 engine, and I have driven it on 30+ degree days without a problem. My engine bay is very similar if not identical to the 409 and I don't have bonnet louvres, in fact my bonnet is insulated.
Best regards,
Kevin
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