I assume by OEM you mean the one for manual transmissions that's still made? This would for sure be the easiest route, but not the cheapest. If that AT circuit really does operate as a cooler as well as heater, I'd for sure need to add a separate AT cooler. I don't tow anything, at least not yet, but I live in Phoenix and my most common trips include steep grades where the ambient temperatures can be well over 110F. I don't think you could add too much cooling capacity.
I'll keep thinking about this. Those eBay aluminum radiators are intriguing and would for sure be the cheapest solution if everything lines up well, but I'm not sure I want to be the subject for that experiment. Theoretically somebody could do quite well for themselves making an all aluminum radiator that was a direct drop in and used all the same hoses (or have custom made silicons that create a drop in solution). It must exist somewhere... too many 1hd engines out there in the world that somebody didn't figure it out, so it's just a matter of identifying it. And that's the trick with all the eBay versions - no product reviews and the sources change so frequently that even if you found a review, it might not be the same unit that arrives on your doorstep.
The stock radiator is god awful as a cooler. You need an auxiliary one. And living in az it's warm enough that bypassing the radiator won't create issues with the fluid not warming up.