You have a 7 1/4 right? My slant has a star shaped filler plug (74 car) and I have never seen one so big. Was your plug like that? What tool do you use to get it out? I use a piece of clear tube and skinny funel to fill my Trans. Works great
That would be a trox bit. Harbour freight has cheap sets as do most places you'll just need to work out which size probably a T50 OR bigger would be my guess. Unless it's the bolt type then just use a 12 pointed socket.