I just put in a Perma-cool 23000 gvw tube and fin type. Seems to work very well. Run your upper line to the rad, then out of the rad to the cooler, then back to the other tranny line. I did it with a 2 sections of brake line, 2 #6AN hose ends and 2 brake line to nipple fittings. I bet total cost was less than 100 bucks. You can see it mounted in this pic. I put rubber pads on the back and used 1/4 inch wide zip-ties to hold it to the cross braces. Its not going anywhere.
Thanks
Coach