It seems to be labouring at over 80 mph
It's not the gearing it's the shape. About as slippery as a house brick.
A trad Morgan is considerably less slippery than a house brick, the CdA with the top down is horrific. Adding another gear is unlikely to help as the existing 5th gear is almost certainly an overdrive ratio anyway.
I'm finding it difficult to imagine that a 2.0l Duratec is genuinely struggling at just 80mph, I suspect you are simply not giving it enough throttle, the Durance is a great engine and will easily rev to 6k rpm. Try using 4th gear and 5k rpm.