Firstly, you should stop driving it. For your cheap fix, fit the 166 gearbox with the diff cover from the GT 'box and it will be sorted. Then, if you've caught it early enough you might be lucky and be able to repair the GT gearbox with a Quaife or similar diff. But usually when one of the diff's spider gears breaks up it punches a hole in the gearbox casing into the bell housing and that's really the end of the box. Do you have the suspicious smell of gear oil leaking from the bell housing area?
All this assumes the diff is your problem, it might be something else, but probably not.
And many of the 156/147/GT diesel cars use the same type of gearbox as the V6 cars, but they're not actually the same as they have a different bellhousing to attach to the diesel engines, and different gear ratios to account for the fact that diesel engines have very different power and torque characteristics to a busso V6.