The Gomog jacking diagram is ok as far as it goes.
However the junction of the Z member and a cross member is a strong point, so lifting there is fine.
The centre of the rear cross member, yes that's ok (Gomog agrees) but I'd not suggest the centre of the front cross member as the weight is considerably more, and there may be a balance issue. Also, not all chassis are fully welded there (Woodstock was - after I rectified matters).
The one thing never to do, is jack on a Z section. I've seen many Morgans with bent chassis rails as a result.
