Rog747,
I joined Britanna in 1979 and recall flying with a captain who had been involved in the stretched DC-8 project. He would have beeb the fleet captain if it had gone ahead.
Just how much momentum or traction the project gathered, what the aeroplanes would have been used for or when they would have been bought/leased I can't tell you.
Mind you, during the 70s (when this would have been happening) Britannia engineering looked after the IAS DC-8s, which came to Luton for their hangar visits.