If you have the factory unit, why did you buy the aftermarket? Or did you not know you had the factory unit?
A two-pass cooler (like OEM) should be sufficient for most applications, you won't be towing five tons often enough to be a problem, and the AW4 is a fairly tolerant slushbox anyhow. If you're dead set on installing the aftermarket unit, then I would go ahead and disconnect the OEMR and pull it out, and install the aftermarket unit in its place.
Then, let me know you've done it - because I'd happily pay shipping for a small cooler like that to use around here somewhere. I haven't decided yet, but it would either be a transmission cooler on her commuter (if I don't decide on something else,) or either a power steering cooler or an axle cooler for me (if I feel the need.)
5-90