Just wanted to post what my experience was. The Wix 58880 filter is slightly taller than the factory filter, at least for my ‘66 ST300. Just long enough to keep the pan from seating. That’s all I got. See my pics for a comparison of the filter that was in my transmission (left) and the Wix (right).
Diameter is the same, the clip that holds the body lines up, it is just about 3/16” too tall, it’s hard to get a picture of the difference but I’m telling you it won’t let my pan sit down on that corner. It was fine with the old filter. The old Wix number from my Wix catalog from 1984 is 51880. I think all of their numbers were 51xxx back then so this 58880 filter should be correct but it didn’t work for me. I’m not upset (I have beer in the fridge) I’m just sharing my experience and appreciate any input.
Might this be an option for you? Otherwise, cleaning out your old screen and reusing is another option. http://www.oldbuickparts.com/product_info.php?products_id=8418
ATP had the correct one tooled about 12 years ago. It was the only one in the aftermarket that fit correctly. ATP was purchased by a Mexican aftermarket company( don't remember the name). Maybe you can find a NOS ATP part.