Depending on who you talk to in the transmission industry, you might not want to go to strictly an external cooler. Some knowledgable types have told me that not only does the trans "cooler" in the radiator help to cool the trans, but also helps get it up to operating temp in real cold weather conditions. I would say that for most street applications, it would be best to stick with the stock type cooler. An external cooler is good if you are towing, or really leaning hard on the car.
Just my opinion
Advanced Automotive Machine
1971 Heavy Chevy - original owner
Team Chevelle #100