Well, here's my 2 cents. Since I don't think you'll be towing a boat in below zero weather, I think you'd be just fine with an external cooler. I have been told by tranny guys that you can't put a big enough cooler on automatic trannies, and that in hot weather, the cooler you keep the tranny, the healthier it'll be. Especially if you're towing.
I would attach the biggest cooler you can buy right to the front of your radiator, and then either run soft lines or bend up hard lines around the radiator to the cooler. Stock coolers inside radiators are not good enough for towing.
