I'll be putting in the new radiator soon and I thought I'd ask you all for some options. I currently have the trans fluid through an external cooler (late 90s explorer cooler) in front of the radiator. It seems to do fine but I don't have a temp gauge/sensor on the transmission to confirm that. The new radiator has the ports for an internal cooler. So what do you think? External cooler, internal cooler or both (see diagram A or B).
Side questions:
The drain plug is plastic. Would you replace it with a brass drain valve?
The cooler lines generally go "IN" the top and "OUT" the bottom?
Thanks guys.

I'd run the internal if you are going to drive in the winter. I read the trans will need some extra heat (from the engine coolant) to bring the trans up to proper operating temp. It might not get warm enough with just the external cooler. Conversely, I wouldn't run the internal cooler in the summer, because the trans can pick up extra heat from the coolant. It would be nice to be able to close a couple valves & reroute the trans fluid depending on the season. But that is probably easier said than done.

I don't have any suggestion on routing. Everything I learned was from reading up on automatics when I was considering the 4R70W. I actually don't have any real knowledge.

Lots of modern cars have plastic drain plugs in the radiator. Is it a pep-cock or just a screw in plug. If its a pep-cock I'd lean towards replacing it, but if its a plug w/orings, I'd leave it.

Use diagram C. It's just like B except the supply lines go to the bottom of the coolers.

Radiator coolers should be on the cold side of the radiator so it shouldn't add heat to the fluid in warm temps.

How warm does your coolant get on hot summer days, especially under heavy load / duty ?

And quit drawing pictures of trannies.