I can't answer for the exact BTU cooling requirement. The real question is how much cooling does the relatively small exchanger inside the radiator provide? It is liquid-liquid so more efficient, but also warm coolant. I would guess its getting maybe 10k BTU cooling at most once everything is warmed up. Maybe a single larger air cooler would be better and simpler.
Looks like you are in a warmer climate, so I don't think warm up is an issue. In cold climates, keeping the in-radiator exchanger helps get the tranny warmed up and shifting better earlier.