I don't know the specs on the OEM heat exchanger, but I would NOT install a new one that has smaller fittings than the OEM that was part of the trailer towing pkg. Bigger is okay, but not smaller.
When you get it all done, you must do a flow test to be certain that you have adequate flow through the ATF cooling system. Even with a huge heat exchanger and big fittings, you could have kinked a line or a fitting somewhere along the line. I don't know the requirements for your tranny, but the 4R100 in my 1999 SuperDuty required a minimum of one GPM flow back into the tranny after the coolers. That was also one quart in 15 seconds with the engine idling. I suspect that's the same for your tranny but I haven't looked at the specs.
Grumpy ole man with over 50 years towing experience. 2012 F-150 Lariat SuperCrew EcoBoost, 6.5' bed, Platinum White/tan, Leer 100XQ camper shell, Nomad Joey 196S TT, CarMate 7x14' enclosed cargo trailer, three other utility trailers, plus a retired 2000 Keystone Sprinter 25RKS 5er we rarely tow now. ProPride hitch on the TT. Reese Strait-Line hitch on the cargo trailer.