Kris,
 In order to order parts for your XJ40, the VIN IS the chassis number.  If
you go back to my 1967 Jag, you will discover that there was no VIN number.
There is a chassis, a transmission, a body and an engine number.  The common
denominator was to use the Chassis number as the VIN.  This is in fact what
my State uses as the VIN in the title.  Some States add the model year in
front of the Chassis number to create a VIN.

   When ordering parts for my 1988 XJ40, they always ask whether it is a 5
or a 6 chassis/VIN number to determine which part to send.  You should do
the same thing.  I RARELY give out the last 6 digits of my VIN since they
are not relevant to the issue of ordering parts

Cheers
Webserve