Unfortunatly the fuel inlet on the fuel pump is right at the bottom of the fuel tank without a filter, so the 1st thing to suffer is the fuel pump! Get under the car and you'll see an L shaped hose to the pump, be very carefull undoing this as they are NLA and not easy to find! Once off look at the connection on the pump, sometimes the pumps have a wee mesh filter here which gets full of crap, carefully remove it and clean out all the junk. Next blow the pipe through and reconnect the lot.
Oh and be ready becasue once disconnected all the fuel will come out, have a big bolt to block the hose or something

Once you've done that and replaced the filler neck if it is indeed rusted, get a magnet with a length of string attached to it, and drop it into the fuel tank. Drive it for a week, pull the magnet & clean it, droip it back in and repeat till the magnet is clean!
If you lift up the rear seat you'll see a round plate under there, take this off to gain access to the fuel level sender, pop that out and drop the magnet in there
