Not so much where I live, only 80-90 degrees in summer and relative humidity is forecast at 90% today. The island I live on is surrounded by rather chilly 50-53 degree water. A quick road trip over to the the valley will net 100 plus degrees. I do live below the 49th.

You may have the original cover for that engine, you didn't mention what year the engine is did you?
Do you know the heat range of your spark plugs?

Like I said, timing may or may not be your problem, by process of elimination hopefully you will solve your overheating quandary. Just as likely to be water, air flow, heat transfer, lean fuel/air mixture or a host of other causes. Not knowing the initial timing setting, problems at idle but not road speed and having a similar problem myself led me to think your cause may be similar to mine. One could just as easy think airflow, due to a hot condition at idle and normal operating conditions at highway speed with more airflow through the radiator and over the engine itself.

One can rotate the distributor and set the timing by ear, it's been done before, but I wouldn't recommend that route.

I think it imperative to verify initial timing. Either find out from someone who knows for sure what the marks on you cover represent or establish a top dead center mark using a piston stop.
You stated the distributor technician set your distributor to 12, 24 and 36. I deduce that would mean 12 degrees initial with no mechanical or vacuum advance, 12 degrees mechanical and another 12 degrees vacuum? Or is that the advance curve? You would have to know your initial setting then check at what rpm your mechanical advance started and ended. Same for vacuum. Can you enlighten us as to why he set the distributor with such parameters?

I have no opinion on the looks of the dual fans, I like the free part, and if they solve your overheating puzzle that's grand.