TH400 is a TOUGH transmission! If I didn't want OD thats what I would be running, I put my 700r4 together to be fairly tough and am running a huge cooler on it.
I'v seen a th400 overheated more than once and it just kept on going, we put a cooler on it after the last time though, I should have been running 2nd gear on the back roads so the converter didn't slip so much, was towing my suzuki samurai on a trailer that probably weighs 3k lbs or more!