If you can stretch the budget to a newer 4/4 with the Mazda 6-speed box, then all the better!
Is there at Morgan a six speed box available for the 4/4, or was it a typo and you mean the 5 speed box? I ask because I'm currently researching if a 6 speed box would fit into my 2014 4/4. After calculating the ratios it could give a better result than a shorter axle....If I should find out that the 6 speed box would fit by size.