Cleaning a filler neck

on my 66 dart, the original filler neck is mangled where the cap goes, so cap doesnt fit right.

I found a used filler neck off a 65 dart at the junk yard, which is not mangled but the inside of the tube is rusty down at the bottom , like that tongue that sticks out. the top of the neck, inside isnt rusty but is oxidizing white. Is there something I can soak this in to clear it all up? Vinegar? WD40?
Any ideas?
My tank is new so I dont want rusty getting into the system if I can help it.
thanks faboers