I would get a second opinion before I started, make sure it is not a bad gauge. Find a friend with scanning software and see if they match. I have a 04 Escalade ESV and tow a 7600 lb trailer with the stock towing package and never see over 185 on a 90 deg day. Going up and down hills I have not yet hit 200. Also, if you get a smaller cooler than 40,000 btu you might run it in series after the stock trans cooler.

 

My factory trans gauge usually reads about 170*, I'm using the stock stall and have a 160* stat, 05 efans set to come on at 180*, and a PermaCool 17k trans cooler. The other day it was well over 90* out and I saw 200* trans temp in traffic with the ac on.