I think you're on the right path provided you locate the new cooler out of the airstream of the radiator/condenser (under the car). Run hot fluid from the tranny to the new cooler first, then to the radiator cooler, then back to the tranny. In hot conditions, you'll knock most of the heat out with the new cooler significantly reducing the load on the radiator. When in traffic (no air moving across the new cooler) your radiator connection will provide enough cooling for you to keep the tranny happy. In the winter, if your new cooler is 'overcooling', your radiator core will put some heat back into the fluid for you. I think that's a very elegant solution if you're not gonna run some kind of t'stat to keep from overcooling the fluid.