I know that Jays, (John) has written here that making the ratio of the rear axle of his 4/4 shorter was the best of all the measurements to make his Sigma 4/4 much more agile and give her more torque (in the shorter moment). So if your diff is possible to be changed...
(I have learned that coincidently the production time of mounting the salisbury axle which allows to change the diff ratios was the same as using the Ford gearbox, so the best combination would be an MX5 gearbox with the 4.1 ratio which no one has realized so far and with the Ford box you could sacrifice the range of the first and second quite a bit)
...why not doing so if you find someone who will make the job. The whole axle shipping to England is may be not such a good solution but perhaps you can drive her to Milan if the times are more normal again or you may find someone in your region when your Morgan shop is hesitating to make the job (BTW it is much better that they are honest because not every shop has the required skills and routine).
I think that your 4/4 is most of the time driving nice mountain roads and you must not fear that the 5th gear will become too short as it is said that the Ford box 5th is too long anyway.
