When buying the socket make sure you get it on a 1/2 inch drive and not larger with an adapter,because this causes a lot of space constriction and hard if not impossible to get the sender loose.
My sender on the w124 300d was bad after previous owner ran wvo for years in car, i had a spair form a 300d,it is in the car but i do not see the needle dropping a whole lot,maybe i get great mileage or maybe its stuck?
You are sure the 300d and 300d are identical right??