Lets put it in perspective. Automatic transmissions are about 80% efficient, so only 20% of the power that goes into the transmission becomes heat. Since most of us do not drive up hill all the time with a maximum load, the average power to maintain vehicle speed is between 15 and 25 hp. So we are looking at the need to cool 3 to 5hp on average with steady state driving. Since the transmission itself is a large heatsink capable of dissipating about 200 BTU/minute, that leaves the transmission cooler needing to dissipate about 200 BTU/minute. (on the other side, the stock radiator can dissipate about 3000-4000 BTU/min at highway speeds.)
A frame mounted cooler would be fine if it gets enough air flow. My recommendations is to use a small oil cooler with a temperature switch controlled fan and a thermal bypass. Something with about about 0.8-1.0 ft sq core area. Mount it under the cab behind the battery with the fan blowing down or inward, or somewhere else out of the way like the apron behind the grill or somewhere.
The temp switch will regulate air flow and keep the trans from getting too hot, the thermal bypass will bypass the oil cooler until the fluid gets over 100-125F warming up the trans earlier.
Just my $0.02
