Vitara to Sierra Diff Conversion

I have an older leaf spring Sierra that we use as a shooting buggy, it travels on a trailer and will never be registered. My plan is to convert it to a Vitara auto gearbox (to free up my gear change hand). I plan to use a modified and complete Vitara rear axle (5.12 ratio). I am after info on what is required to alter the front diff to a 5.12 ratio, I have an old Vitara front diff centre, I understand I use the crown and pinion gears but what other tricks are required? I am sure this would have been covered in detail previously so if you could point me in the right direction it would be much appreciated. Cheers.