[Gromit]

That idea sounds promising to me Nitis - I'm not familiar with the other years but here is a photo of the 1970 EEC sending unit for what it is worth.. Mainly to rule it out I suppose but just to show the configuration. The vents are higher up and on each side of the tank.

An observation in favor of a sending unit modification (or retrofit with a later year) like Nitis suggested is that the height of the sending unit is lower than that if the upper drivers side tank vent - and importantly perhaps even with or slightly below the level of the carburetor fuel rail.