There should be a rubber O-ring around the fuel pump. Between the fuel pump and tank.
Put a little grease around the O-ring. That will help the ring turn to lock.
Yes it should be a little hard to turn since you have to compress that O-ring. Once you get the ring started in the locking tabs. Get a brass hammer or solid rubber hammer. If needed you can use a brass punch against the ring. And tap with the brass or rubber hammer.
Reason for rubber or brass is those material don't cause sparks. You don't want sparks around the fuel pump or fuel tank.
Let us know how it goes for you.
Send pics.
Greg