Trans cooler questions

I normally run manual shift trannys but am putting a 42RH in my sons car. Doing a lot of mockup before dissassembly to finish the metal work. The trans uses 3/8" lines. Looking at different coolers. Thinking the plate / fin cooler is what I would like to get because of cooling efficiency per size. Many different ones some rated by extra towing capacity, some rated by BTU radiation per hour. It's all confusing for me since this isnt something I deal with normally. Not sure if bigger is always better in this regard. Car will be around 300hp 318, going to look for a higher stall converter and install a shift kit, deeper sump pan. Rear end gears will be 3.73s. Basically a fun street car. Theres one I was looking at that's 8.5"× 13" × 2" and 46,000 BTU per hour radiation. Rated for 20,000 lbs towing. This car wont be pulling a trailer obviously. Am I overthinking this? What size do I need?