I've got the v-8/v-10 (both engines use the same cooler) tranny cooler and an auxillarly cooler also. I've got a pic in my webshots showing size comparison between the v-8/v-10 cooler and the standard 7.3 sized cooler. Click on the link below my sig.
When I was towing & traveling for a living, I think generally the summertime temps were around 150 - 175. The highest I ever recall seeing was about 195 for a brief period. Non-towing the temp gauge hardly comes off the peg @ <100.
Since the 6.0 came out, the more popular mod is to install a 6.0 tranny cooler instead of the v-8/v-10. It is even larger than than the gas engine cooler and will still bolt right in. The main issue with doing this sort of upgrade/mod will be getting the cooler lines hooked up. You may have to do some fabrication with some supplies from the local auto supply or hardware store. The last I heard, that is what
BRIANS TRUCK SHOP does when they do their tranny mod magic.