Says the guy who owns a Jaguar! Your XJ was produced from 1968 to 2003 with little to no improvements.
Uh, no.
The 2003 XJ shared NO parts with the 1968 XJ. They weren’t even remotely the same car! Not only that, but the original car had dual fuel tanks carried behind the rear axle and from the XJ40 onwards (1988 ‘XJ’) it had the tank ahead of the rear axles in the passenger cell area.
For that matter, the last of the classic XJs in 1987 only had about 40% parts commonality with the first XJ and most of the remainder didn’t interchange at all. Jaguar made continuous improvements to the XJ from Series I-III, then the all-new XJ40 came out, followed by the heavily revised X300 and the again revised X308, the ‘last’ pre-Callum XJ.
Nice try, no points, come again.
After installing a five row radiator, an huge trans cooler, having the GM (!) Transmission shift points reworked, and fixing a stuck button on the 8-track, I was able to drive my 1976 Jaguar XJ-6 many hundreds of miles between overhauls, with only minor daily electrical problems.
All of this mind you, while achieving a very green 15 mpg. The R-12 AC (borrowed by Jag from GM, along with the Trans and window lift motors) worked perfectly. you can't beat it! Still have it. Somewhere.