Gearbox upgrade

[cav773]

Hi Guys. Perhaps someone far more knowledgable than I could help with a problem I have.
I have located a Mini GT1275 gearbox remote change type. I wish to use on a 1275 A+ engine. I am ok with the remote change side of things but what mods will I have to make to the GT box considering the difference in wheel diameters 12inch to 10inch on the Mk1 I want to use it in.
Thanks in advance for any help given.