just had a quick look at the Haynes Manual and the auto box uses a cable select so that should be a very easy option, you just need to find a long enougth cable to do the job, Speedy cables could probably make one up or you may be able to use something like a brake cable.
The cable has a threaded inner at the selector end and either a fork at the box end or a clamp (similar to those on the throtle at the carb)
I'm guessing you haven't got a selector yet, and I'm not sure how easy they are to come by. You may be able to find one from a different car to use so long as the positions are the same and the distance between them is the same as the mini too.
Simon