I'd be inclined to plumb a trans temp gage into an external bypass line to see whether I needed a cooler, or not, without sacrificing a transmission un-necessarily to find out.
External coolers in the radiator started as a Heavy Duty option on some cars of the 50s/60s. Standard was air cooled via holey Bell housings and finny torque converters.
https://s-media-cache-ak0.pinimg.com...389a122d8a.jpg
http://forums.aaca.org/uploads/month...3142564952.jpg