First thing I did was landfill the stock transmission cooler on my F350 and replace it with a Hayden 12 pass swirl cool cooler, then I landfilled the stock sheet metal transmission pan and replaced it with a cast aluminum, finned B&M deep sump pan that adds an additional 4 quart capacity. The B&M pan comes with a temp sending boss cast in.
None of the FoMoCo factory cooler are worth much and a slush boxes main killer is heat and foaming fluid. My box never exceeds 210 degrees, no matter how hard I'm pulling in 2 or 4 wheel drive in any range, on road or off road.
The B&M pan makes fluid changes super easy too. My fluid gets changed every fall no matter how many miles I drive. Every 2nd year the pan gets dropped and a new filter installed (with the fluid change). Dexron Mercon V comes in 5 gallon pails btw.