Standalone trans cooler vs radiator cooler: which way keeps an automatic alive?

A standalone (external) cooler. Why a separate cooler: higher-stall converters make more heat at higher rpm, and the cooler in the radiator tank often isn't enough. It also dumps that heat into the engine coolant, so if the engine runs hot, moving to an external cooler helps both. Speedway's 4L60 guide lists a large external cooler as a key upgrade, since heat is what kills that transmission.

Mount it where it gets direct airflow — Speedway mounted theirs low in front of the radiator, lined up with the grille opening. For a higher-stall converter, towing, or a 4L60E making power, the external cooler is what does the work.