3:31 is a VERY common Jag rear gear ratio, not at all surprising and a good overall choice. If it's from an XJS it will (most likely) be a limited slip type rear, again an excellent choice. XJS rears are preffered over the XJ6 sedan type rear because the XJS is a little bigger, a little stronger.
We may be done here!
I WOULD have some concern when running a typcial 3:31 rear gear with a CLOSE RATIO Top Loader because 1st gear is a higher ratio than found in the WIDE RATIO Top Loader. Making the overall 1st gear ratio possibly to high for a small block or a 'hilly' area. 4th remains the same in either trans.
Me? BIG BLOCK, 3:31, CLOSE RATIO Top Loader, love the combo!