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A lot of US-made cars equipped with M-block (351M/400) engines also used the FMX transmission up to 1977 or 1978. Most M-blocks had the big-block bell housing bolt pattern. There were relatively few M-blocks with the small-block bolt pattern made in the 1973 model year.
So I would say the FMX with a big-block bolt pattern is not very rare (in the US anyway).