Ok cool! I don't plan on working the truck too hard. Just don't want the "milkshake" and high temps while in summer traffic.
As Tim already mentioned, completely bypassing the radiator may not really lower your transmission temps in certain conditions.
There are two schools of thought here: first is separating the tranny cooling from the engine cooling systems. This ensures there will be no pink milkshake but the trade off is ( in my and others opinion ) a less well-rounded and capable tranny cooling system.
The second school of thought and the one a few of us subscribe to, is to run the cooler in-line with your radiator. This does not eliminate the possibility of a milkshake, but if you replace the radiator at the same time ( or you know the radiator to be fairly new and in excellent condition ), you wind up with a more effective and well-rounded cooling system while eliminating most ( not all ) possibility of a pink milkshake.
Now let's be honest here, most of the pink milkshake stories I've seen are due to lack of maintence. By that I mean the radiator was never replaced as part of the truck's routine maintence. So eventually something fails in the radiator. Sometimes it's the radiator itself — sometimes it's the tranny cooler. Waiting for a radiator to fail is just plain dumb. You either cook your engine or poison your transmission. And for what — a radiator is $100-150! Even I'm not that cheap.
The solution is simple; replace your radiator! They are cheap as fu€k and simple as hell to replace. There is no good reason not to replace the radiator if people are concerned about the milkshake.
I determined that my radiator was functioning perfectly fine — I still replaced it though as routine maintenance and part of the cooling system overhaul I just did. I'll replace it again in 100,000 miles or so when I hit the half million mark.
I personally think there is more risk in repeatedly running the tranny fluid at higher temps, than the possibility of a pink milkshake. Heat is to me, more detrimental than the
possibility of a pink milkshake — which is mostly preventable. Replacing the radiator as routine maintence, adding an external cooler in-series with the radiator and changing your tranny fluid either by DIY slow-flush or regular drain & fills, will probably be some of the best things to do in terms of aiding transmission longevity.
I should mention however, that most people do none of those things and their A340s last an easy 2-300,000 miles.