Personally, I don't think you will need an additional tranny cooler. My 2013 Chevy Duramax/Allison has yet to run out of the safety range on the engine or trans. I towed 10,000 lbs of TT up a long 14% grade that I thought would never end on the Blue Ridge Parkway and saw 200* on the tranny which is 50* from max. On 5-7% grades, the trans temp was around 185*. It generally runs around 165-170* on level ground.
I think you will find the stock cooler to be all you will need.
2013 Silverado 2500 HD Duramax/Allison Tuned
Aviator Wright Flyer #1919
"First In, Last Out". 11B4Y
DAV, MOPH, VFW, 5/7 Cav,1/8 Cav Assoc.
11th ABN Pathfinder Co. N7MIP