First a warning: The seat runners are bolted to the floor by bolts going into loose nuts that fit into a steel loop welded to the floor. Undoing the seat with its runners is easy enough, but replacing it requires these loose nuts to be held in place with a finger or something, while the bolts are started. To undo the bolts push the seat all the way forward to get to the rears, and all the way back to get to the fronts.
You then have two basic choices: Weld new loops for the rear and front bolts, both further back, or fab up some extenders, in some way or other, which are either bolted or welded to the old loops. This is a safety critical fixing, so it must be as secure and solid as the originals.

 

Unfortunately you live in Australia and modifying the seat mounting points requires an engineers certificate to be legal. See page 14

 

Here is a link to XJS Parts. I've been thinking about getting them; but they are pricey ($325). I'm 5'11" and feel a little cramped.
This is the add: "Especially developed and designed by a 6' 2"driver owner of the XJS during 14 hr long painful trips. The extenders are sold as a set for both seats and increase your legroom from 6" to 10" without any modification to your seat rail or car in less than an hour. You simply remove the four bolts holding the seat to the floor, install the extenders to the floor (bolts and instructions included) then bolt the seat and rail to the extender, and enjoy your XJS like never before in great comfort as drivers not as tall. I am so happy to share the comfortable experience that I have enjoyed, for the last 10 years. The Set is made of solid aluminum then powder coated to match the original seat rail. For Convertible or Coupe. Patent pending US 62/548,333"

$325 USD

https://jagxjsparts.com/seat-parts/s...ender-package/

 

These would still need to be engineered in Aus to be legal as they are not OEM. This could be an issue in an accident where the Insurance company could void insurance because the car does not comply with Australian ADR's or have approved engineering compliance.

 

Hmm. "... increase your legroom from 6" to 10"... I don't know about you, but there's an inch at most behind my seat when I'm in it. Adding even six inches would require removing the rear seat. Unless this is measured in a weird way, like a curve.
Also, those are chunky slabs of metal--my hair tends to brush the ceiling liner as it is. Those might rob me of what little headroom I do have.
And they're damned expensive for metal bars with holes drilled in them.... (And why would I need both seats? No option for just the one seat?)

 

SDSD
The later models have far thicker seat backs, and maybe as well the seat bottom is a bit longer. My 1985 with the early type seats has far more room behind the seat, and far more seat travel, than later cars do.
As for the metal bars, myself I would not trust aluminium for this safety critical task, but make something out of steel at a cost of about 5 UKP! (if I was outside Australia, obviously!)

 

Hmm. All that lumbar support and mechanical fluffery inside making them thicker, I guess. How close to the rear seat does your '85 get?

 

In my 95 conv, the windshield always seems to be lower than what I'd like meaning at a traffic light, I cannot see the hanging light. I have to slide down lower to see up high or even just straight out. I'm always looking at the top of the windshield frame more than out the center of it. I do feel that either the seat is too high, or windshield frame is too low or I have a long torso, (which I cannot do anything about except try to recline the seat back at more of an angle. I'm 6'1"). Also the the instrument indicator lights on the brow are not at an optimal angle either if one sits high.
...But I'll take all that nonsense just to enjoy the ride it gives me.

 

Hey, at least you have a drop-top - you can always look over the top of the frame! In my coupe, I have often had to crane forwards. Not at most road intersections, as the lights are on the far side, but for pedestrian crossings? Not a hope of seeing those lights without craning.
And at least we're not 6'4" Tom Selleck trying to fit a Ferrari--he had to have the seat bottom taken completely out to get him that. Sean Connery tooled around Japan in a custom-made Toyota 2000GT convertible because he literally couldn't fit the standard coupé.

 

I am 5'11" with quite long legs in that total. If I have the seat all the way back I can barely touch the pedals; I drive pretty straight legged, seat not fully back at all, and like that there is a good 4 or 5 inches between the seat back and the rear seat cushion leading edge. A notch further forward and I can (and have for a long journey) sit in the back no problem. I think it is the Recaro-type design of the later seats, not the stuff in them, that is the difference. I drove a 1989 3.6 once, it had the later seats but no electric or lumbar stuff, and it was impossible to sit in the back even with the seat as forward as possible.

 

The mind boggles at the difference in room. I'm two inches taller than you, and with the seat all the way back my left foot rests comfortably on the sloping part of the floor behind the pedals (don't need a dead pedal). And there's about an inch of room behind the seat. I cannot sit in the rear seats not just because of the lack of knee room, but because I would have to be decapitated to have my torso fit upright.
Pity I can't just swap the seats for the older type....

 

I had to fit a dead pedal as I got fed up with my foot dangling, which can be seen in this photo. I had to hack about an inch off the brake pedal to make a comfortable gap for my size 10s!

 

Blimey. That's some difference. Tomorrow I might measure a few distances with a tape measure. See how much space the newer seats gobble up.
Cutting a chunk off the brake pedal is something I've actually been tempted to do--it's not too bad in boots, but in sneakers, especially some, it's a tight squeeze to get my left foot in place. I'd actually like to shift it an inch or so backwards as well, but I suspect that would play merry havoc with the brake feel (travel before full braking) even if it were possible physically to move it.