SIII is the best for that.

Yes, command cruise from K-Mart.  Have a look at my website.
http://home.iprimus.com.au/rmcgrice/Cruise_Control.htm

Ron

I found this interesting...
http://www.jagweb.com/aj6eng/xk_performance.html

 You'll find a thorough description of the SIII fuel injection at
http://www.jagweb.com/jagworld/42efi/page3.html
    The SIII constant energy ignition is a worthwhile update I
believe, it uses a GM ignition module, you'd need the SIII distributor
as well.
       Changing to SIII fuel injection however is not a task I'd like
to take on though. The system is controlled by a computer in the boot
so a lot of re-wiring would be required, including the fuel pump and
relays, etc, etc. You'd have to change the fuel tanks too, (if they
fit) and lots of plumbing, the SIII has vapour separators in the roof
pillars and the charcoal canister in the right front wing. The cooling
system may be different as well, different header tank setup.
       An EFI system returns excess fuel to the tank which is fine if
you have only one tank, but an XJ has two as you know and the SIII has
one fuel pump, a change over valve to select which tank fuel is drawn
from and two return valves. This arrangement is notorious for rawing
fuel from the wrong tank so you run out of petrol and/or returning
fuel to the wrong tank causing it to overflow.  When the starter motor
on my SIII failed a few months ago the car had to be towed, I could
have removed the SI starter in 10 minutes from above. Somehow I think
triple carbs and extractors would be easier, maybe change the
transmission to a 4 speed.
Regards,
John Hudson
Brisbane, Australia
1971 XJ6 SI
1985 XJ6 SIII