I'm just running an external cooler and it works fine. I also had an external cooler on my Jeep truck and pulled many a trailer with it and never had a heat issue.

I just remembered the other day something that happened to me in the '70's. I had a Pontiac station wagon with a 455 in it. One day I took off the radiator cap and it was like a pink milkshake in there. Long story short, the transmission cooler inside the radiator fell off and the transmission fluid was pumping into the radiator and the coolant was in turn getting sucked into the transmission.

I tried to save the transmission by repeatedly flushing it after I put a separate cooler on it, but a short time later it started slipping really bad. Had to have the transmission replaced. One more reason I don't like to use the radiator as a cooler for the transmission.

Don