I have had a whole heap of advice over the last few days from a bunch of different sources and the issue is that there are several different parts that changed from year to year, not just model to model. In my case, the car is a 71, the gearbox, diff, etc, etc, could be from anywhere. I will see what I can find, but if I can get what I have to work all the better. Otherwise I could be banging my head against a brick wall for ages trying to source all of the different moustache bars and diffs, etc until I actually find the right combination that fits.
As for the 'oblong holes' to make my diff fit in the moustache bar. It is actually quite a tight fit. Alignment could be slightly out, but time will tell. If I have a heap of vibrations to deal with, then I will sort it out.