most if not all the vibration/broken stud problems i have encountered over the last 20 or so years of diff conversions have been wrong wheel bore size . i agree that the older 70's cars had no center location , guess what the first thing you do to a 9inch axle is ? any thing that has the grunt to turn the tires will break the wheel studs if the stud is the only location , not trying to push the point but i've seen it too many times to not take notice of it , and i would have thought that a dirt rally car that is often off the ground and ploughing through ruff roads would want as much in your favour as you can get