My XJ6 has all 4 headlights the same size. I see photos with the outer 2 larger to fit the hole they're in without the trim that fills the void around mine. Can I retrofit this on mine fairly easy? What is required?

 

Yup!

The 7" headlight conversion requires the appropriate trim ring ("embellisher" in the parts catalog, as I recall) and a 7" headlight bucket and retainer ring from virtually any Brit car: MGB, Triumph Tr6 or Spitfire, etc.

The actual headlamp is your choice: Hella, Cibie, or even an ordinary sealed beam.

The embellishers are discontinued from Jaguar but come up once in awhile on Ebay.

As it was only the USA cars...and for a certain period, Canadian cars as well, I think... that had the smaller lamps, the larger lamps are commonplace elsewhere in the world, so Euro/Australia/UK ebay is a good option for the embellishers if you find a seller who will ship overseas. Some do, some don't.

Some UK salvage yards will ship oveseas. Do some Googling.

There was also an additional clip used with the 7" embellishers. As the seller to include it but, even if they don't it's no big deal. The screw alone will do a satisfactory job of holding the embellisher.

Cheers
DD

 

Looks like you can buy some of it new from this site.
Jaguar Headlight XJ6 XJ8 XJS XK8 S-TYPE X-TYPE XJ40 X300 X308

A UK supplier has many of the parts you need and also a pdf file with a drawing of the two sizes (7" and 5.75")on page 225.

SNG Barratt - UK | Parts