Rory, I recall you saying 4 years ago that the car was your parents' car, much loved an one of the Plus Eight Lightweights.
As there were only 19 (if my memory is correct) of these cars produced, I would consider carefully a change of gearbox, particularly a non-Rover gearbox as it may undermine the car's value.
An additional consideration is that you mentioned to me in 2013 that your parents bought the car new.
Obviously, it is your decision as to how you wish to deal with the gearbox but thought that I'd respectfully offer some thoughts.
Finally, I converted my 1976 Plus Eight from a Rover 4 speed to a 5 speed in 1988 and am happy to guide you through the changeover.
