After checking the Ford parts catalogue and asking a friend of mine who has owned a lot of MK4 and has an EFI one right now I know a little more.

The inner wings and therefore the header tanks were modified 1989 when the EFI CVH was introduced. The non-EFI header tanks (sitting on the drivers side) won't fit the pre-89 cars/shells as these tanks are higher. Funny thing is the EFI header tanks had the same shape as the pre-89 tanks (the ones mounted to the right / UK drivers side). Only difference is the position of the hose connectors. These sit slightly to the left or the right side but the hoses should be long enough.