Hi again, Have you got a castellated ring nut at the top of the seat pillar tube, that also secures the removable ‘wishbone’ top chainstay section of the frame ? After 85 years or so they can be difficult to remove without getting brutal with them, but once that is off you should be able to get plenty of penetrating oil down inside the tube, and be able to get the tank moving a little bit at a time. It’s a good few years since I last had a fight with one, but it did come off after a lot of swearing, and hitting the top of the seat pillar tube backwards, forwards, and sideways with a rubber mallet, to break the grip of the rust between the tank and the tube.
Brian