You might also want to look into a tranny cooler that has it's own electric fan connected to it. Then it will cool whether you're moving or not. A good comparison for aftermarket coolers is GVW (Gross Vehicle Weight). Flex a lite sells a really big one with a GVW of 32000. That should handle about anything you could throw at it.

A larger tranny oil pan will help keep the temps down as well as there will be an extra 2 quarts of fluid to disperse heat.

A switch to ATF +4 was recommended for truck trannies in about 98 I believe. And All 2nd genstrucks could be switched to+4 and was recommended to dealerships that trannies be flushed and replace with +4. ATF +4 is a semi-synthetic that conducts heat better and won't break down or foam at higher temps. I'd recommend a flush and switch out to that over a+3 with additives if you're constantly running hot, just ask you'relocal mechanic or dealer if it can be donein your tranny. You'll have to pay for the flush, as +4 doesn't play well with other fluids or additives and you'll want to get all the old out.