Welcome. In hopes of getting you started on the right foot, we prefer separate topics be posted in separate posts. Why, you ask, because when, 3-4 years from now when someone does a search for “gaskets” they won’t find a post about gas tank sending unit removal.
To answer your questions the blue looking donuts are indeed exhaust pipe to header gaskets. The other is a seal of some sort. If you google the numbers on it you might get lucky and find the application.
As for the sending unit screws, if they are heavily rusted your squaring and vice grip method might work. Lots of PB Blaster or Kroil.
BTW, gas tanks are readily available new along with new sending units. Relatively inexpensive and can save you headaches down the road. Good luck
