Are you in a hurry to fix the car ? If you are not then I suggest you start/complete the removal of the tank . After swearing/fighting/scraping of knuckles and other " fun " you will quickly tell yourself I'm not doing this again soon

. And you will order the OEM tank .
Or you can rely on the wisdom of this forum ( there is LOTS ) and just order the OEM tank upfront . On these cars the wisdom say's OEM water pumps and overflow tanks are the way to go . Good luck and keep us updated .