Hi I’m from South Australia I have an 1961 s series that needs tlc

As long as the flywheel center hole fits the crank register properly, everything else should work. 10mm is about .400 inches. Was the new engine from a standard trans car or automatic. If automatic, Check the small diameter hole in the end of the crank. Not all of them were finished for a pilot bushing. Usually they were drilled, but not reamed to finished size, and it is possible though not probable it is not deep enough. It would be more likely the pilot bushing is too tight. There used to be a reduced OD bushing to fix that problem, but it has not been available here in the states for a few years. Two ways to fix that problem (if that's what is wrong). Turn down the OD of the stock bushing to fit properly, or use this part. More Information for NATIONAL FC69907
I agree about removing the clutch, and trying to fit the trans up to the engine.