i got the only one of that size, b ut if you search ebay under "transsmision cooler" and set the max of 30 you can find some, either 12x3- 12x5 would work good along the top of the frame rails
Hayden # 401 tranny cooler is the perfect size to fit under the hood on the frame rails. You just have to bend the fins a little to get past the hood mounts.