Also of note, is that although the oil cooler mounts to the radiator, it's a separate unit (or at least was on my 4.5). On the 3.5/4.5 (and presumably 6.3) it's on the RIGHT (passenger) side of the engine bay.
What I would suggest to you, would be to use an aftermarket oil cooler if you can't find the proper-sized MB parts that will work with a W111 in this application. The upside to this is if you have a U-Pull-It yard nearby, you can go in there and see tons of different sizes and applications of oil coolers (including trans coolers that you can use for this application as well), and find one that can mount in front of the radiator or in another location perhaps in the engine bay or elsewhere.
You can also use universal kits that are surprisingly cheap:
https://www.ebay.com/itm/30-Row-10AN-Universal-Engine-Transmission-Oil-Cooler-7-Fixed-Cooling-Fan-Kit/262780853733
https://www.ebay.com/itm/30-Row-10AN-Universal-Engine-Transmission-Oil-Cooler-7-Fixed-Cooling-Fan-Kit/273021712175