Auxillary Transmission Cooler for 1980-90 OCC

I want to add an ATC to my 1990 OCC but I can't find an OEM one for the boxies. I saw the package in my GM Parts Book but I haven't come across one. Will a whale wagon or 9C1/Impala SS tranny cooler work with the boxy wagons? Are the tranny lines different (i.e. SAE vs Metric)?

You can use any trans cooler you want. The fittings are metric. There are adapters for changes in line size. You can use "connectors" to add more line to your original lines. The 91-96 coolers end up behind the bumper, so you would be wise to put a fan on it. My fan is a stand alone unit with a temp sensor on the radiator grid, and a relay directly to the battery. You can bypass the cooler in the radiator and make lines that go straight to the stand alone cooler.

You wouldn't happen to have an LT1 cooler?

I believe I have a spare. I will have to check my stash. I do not have the lines to plumb the fluid. Most of the aftermarket units are less expensive than a used OEM unit unless you get it from a pick and pull junk yard. You still need a fan and control unit for the OEM cooler. You must make a mounting bracket for the aftermarket unit, and I can not guarantee that the OEM unit will directly mount on the older core support, but it should be close. If I have one, it will be $60 plus shipping.

The fan that I bought is rather small and designed for use on a motorcycle. I had to cut large mounting flanges off of it to allow for maximum airflow over the tiny radiator. A larger area unit is easier to find a fan to mount on it. This is the fan I used.
https://www.ebay.com/itm/314317568763?hash=item492ec97afb:g:oe0AAOSwwXBjvoAj&amdata=enc%3AAQAIAAAA0AjXaoxuyNMGZPXcv1yVbwFlstdXXFBBZv3hF8YbPUPpZIa4HGqWDbQ%2FLCtLLHJBX6sXh2C%2F7Lk53%2BalBphoFD14SdtGiCvCoU6UFXuim9kcmnWothxnNunOBOewV5p%2FGkdBjRqJ37bq0GhmtCDxtR4bLNhA%2FFEbP7YGdno5AxBeDiupLSVo%2B4fVAnArq1n4FArUuvmU%2BiMZ7kYhg8e8Uiw5iusUKX2A8ic8QIs42Vdkk3%2FCilLSPSSdFknwvBga30UsS74ugA0jB06IY%2F%2BAATI%3D%7Ctkp%3ABk9SR9CVw7uEYg

I cut the large mounting triangle lugs off of the sides, and mounted it with the through the radiator nylon ziptie style parts.
https://www.ebay.com/itm/321298502879?hash=item4acee214df:g:vH4AAOSwUKxYlNVH&amdata=enc%3AAQAIAAAA8PONni7W7j8%2FkbtqQDMVJGkGsZLOxeJ8Myvky%2FytD6rQJrNiVMliLIToUD%2FvSUQbZChU2qdlJj%2BhU8ojlsyrKG76JjDw60JIpqCmCMH980qgv%2Bm8sv%2BT555cRWvGpl1f6SuJdvcsDyfay3iR3UuUwxF1R9WE5vyJ%2F9sx%2BPLprB%2BsmUopUfAYr5ig1EdEzsz4IomqPEik%2BpTmtP6cj8%2B4M0obX%2BoyIIT6TqvFmG3viK7AqGHqBLyKohoZzbI%2BRPaZvsoR6IM7cbXbc9M4aRKINKY6zIyLsHBJZZQIB7hsUxr4N%2BmfV4T6IFf3XR8LcZFjuw%3D%3D%7Ctkp%3ABk9SR8Km2buEYg

Here is a fan sensor. There are various temp sensors available. You can make it a stand alone automatic system by hooking it up directly to the battery. Hot lead to the relay contact, and energizing coil. Ground the coil through the temp sensor, and output from relay contact to the fan, and to ground. Make sure the fan turns in the correct direction. Use waterproof crimp connectors and a waterproof relay.
https://www.ebay.com/itm/332829025143?_trkparms=amclksrc%3DITM%26aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D249160%26meid%3Ddc9dd5f29474458fb6faf3ecf92035c8%26pid%3D101195%26rk%3D3%26rkt%3D12%26sd%3D234201363911%26itm%3D332829025143%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DSimplAMLv11WebTrimmedV3MskuWithLambda85KnnRecallV1V4V6ItemNrtInQueryAndCassiniVisualRankerAndBertRecall%26brand%3DAmerican&_trksid=p2047675.c101195.m1851&amdata=cksum%3A332829025143dc9dd5f29474458fb6faf3ecf92035c8%7Cenc%3AAQAIAAABUA2rugFlOq3qu1cLac%252F%252Fk6Vp0Oa0HaJIqoXKeIiOR%252BTUgsSvHaeyPxKYu6UqHqq7GaGyKVqHQnjeiiXcQpMGw2t3aB%252BssGfjtIWOBj8wExc7oYYP7xGMyQCrHDyDaSaWjB1CueI3A94n0yxXX5dx5gDvPtdggljJTPuM2g98cBRS6JbDUNrl%252F%252FO7ejxyPMV8jtGwYDGG2AH03MSb6euH309NSthZOzcwG7Py7wG15NtR3IoJsdxHtcrv4nXgv%252BTbR1X6jh%252BzdMojyLNxVxBRmrG53Afo1JFBwGkiUY2tk4l%252B1rncmlMLfF8cxprdFLXbk6DOB5ngXErnxsSZ7q875ibfDTuTrjSlIXWMf8pr3V4uUfKqBi4TzKphnvH13aWvCh2hqRAPn2yW70EldaNRV7C6jLubA56fwu2bABovaNmgX5PmIHA0f6NsU67qLzyuiQ%253D%253D%7Campid%3APL_CLK%7Cclp%3A2047675

Do you have any pictures of your setup? I'm trying to visualize it.

Finally got the pictures.

I think I'm going to go with a Hayden aftermarket cooler with sideways connections and fabricate a bracket. It will be easier to use adapters and bulk line and I can mount it higher above the bumper.

It doesn't really matter which cooler you use, just make sure it has good airflow over it. You can use bulk line with any of them, and connector adapters are available for any variety of sizes.