The forerunner to the M3W, the Liberty Ace, had a shaft drive via a Honda Goldwing (at least I think it was a Goldwing) shaft and swing arm. If you do a bit of searching for photos of its interior you'll see that the transmission tunnel was set at an angle so that the back part of the shaft could clear the tyre. What this meant was the passenger side of the cockpit was narrower than almost any human being and that it would be almost impossible to economically re-engineer it for right hand drive.
Yes, this was certainly a crude design, but it is possible a shaft design could have been engineered more effectively without eliminating passenger space. My assumption would be that the final Morgan design was probably the cheaper option (and possibly lighter)
