I started to fit my diff together, measured the axles and checked the axle length and found the diff housing has been modified sometime in its life. The offset has been altered, so it has gone off to be corrected back to what it should be. After researching the diffs for these cars I have found the most likely diff housing used was a 1959 ford ranch wagon diff housing as this was one of the very few 9 inch diffs with 2 inch wide shoes on 11 inch back plates and drums (which is correct for FIA #1250). This model diff was also a perfect fit into a 63-65 Falcon or 64-66 Mustang.
The diff head was a Detroit locker 31 spline which I have now. So it all should come back soon correct and I can bolt it up.