[David Hodgson]

Wombat, you are correct, most Crowns in Aus in the 70's were assembled here and following the 'local content' requirements, had Banjo diff's. These used the Holden centres and splines but Ford stud pattern, so there's plenty of ratio's available, and would be the cheapest and simplest ratio change.
Having said that, if it see's only cruising duty you should be OK, after all your current one has obviously done OK. Won't last long though if it gets much more than 'spirited' driving.