HotRodN, you want to run the trans cooler in the radiator, but the reason is not so much a space consideration as it is a control feature. Your trans needs a relatively constant operating temperature, and while Houston will see more hot/warm temps than cold it still gets cool to cold for a month or so in what we call "winter". An external cooler works using ambient air, while the in-radiator cooler will be affected by the coolant temperature, warming up quicker and remaining at a more constant temperature year round. An external cooler alone can over-cool the fluid, which is not good. Accessory coolers should be run in series with the in-radiator cooler, and IMO should be upstream of the in-radiator cooler, not after, and are only needed for towing or other high load conditions.