How can the ATF be warmed in the radiator cooler? The ones I've seen have the cooler in the cold tank of the radiator. Check the temperature of the cold tank in your car's radiator on some chilly day. It never gets very warm, and certainly not very soon. The hot tank of the radiator is the temperature of the water leaving the head, 190°F or so. The cold tank is much colder on a cold day.
Where will you get the thermostatic valve? I think a temperature of about 160°F would work, and I don't know where to find that.
Look for a cooler made by Dana's Long Mfg. Co. These are sold under several labels: Tru-Cool, Tekonsha, B&M, maybe others. The tell-tale is "Made in Canada" on the cooler. These have an internal bypass for cold fluid.