Bonzer idea - get your kit from Flowkooler, Hayden, or someone else like that. B&M has been on the decline since it was bought out by a Korean outfit years ago - unless they got independent again.
Most of the external filters I've seen will use a common spinon filter (usually some variant of PH8A or equivalent) and the better ones will have a thermal valve to allow for more rapid heating by bypassing the cooler until the fluid hits op-temp.
A quick note - make sure to setup the mount so the open end of the filter will be up, and ALWAYS fill the filter prior to installation! The PH8A is good for a full quart, and the "shorty" PH16 for a half quart - don't stop until it's full! A half quart or one quart air bubble will wreck a transmission in nothing flat!
Since the "filter" in the AW4 is really nothing more than a strainer, adding a true filter isn't a bad idea. You can get "prefilter" inspection screens that are finer than the sump strainer, and go in between the filter and the mount for inspection purposes. I believe magnetic variations can also be had with a little digging...
If you are going to do this, I suggest you go the full route - get a filter mount with cooler ports and a thermal bypass, go to an external cooler, and don't use the intarnal radiatior cooler anymore - that will improve cooling for both your engine and your transmission. Think about it - does it make sense to run hot oil thru the coolant for an engine known to run warm in the first place? Bypassing the internal cooler was good for 5-8 degrees' reduction in engine operating temperature, which helped to decrease NOx emissions while having no impact on HC and CO...
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