I think that with an engine of 900cc then a 3.62 is much nicer to use everyday and the drop in acceleration is only slight as you will find that you will be doing approx 1000rpm less for the same speed as with a 3.92(this is what i found on a zx9 engine).
There was a company called kit fit who supplied a lot of ford stuff dont know if they are still around.
The guy that used to own formula 27,steve porter,started dealing in engines & freelander diffs,i think they were around 300 quid(ish) new.He lives in Stroud ,Gloucester,maybe worth a search on google etc,or ebay.
The sierra diff ratio is usually etched into the rear of the large gear if you remove the back cover plate.
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