Gorgeous cars. I've loved these cars since i first saw one, 50 some years ago.
A friend of mine, a few years back, bought an XJ6 with serious mechanical and electrical troubles. He replaced the engine, every electrical part, every wire and connector and every hose with comparable GM parts. In effect, he had a Chevy with a beautiful Jaguar body. It was a lot of work, but what a neat car. I should have bought it when he offered it to me.
My memory is that the Jaguar even used a Hydra-matic 400 transmission, making the engine swap relatively easy.
We serviced a couple of late 70’s Jaguars in the early 80’s in the shop where I worked, an XJ6 and an XJS V12. Mind you these were first-owner cars, meticulously maintained. They were a pain to work on-even fueling them was a chore, with twin filler necks-but they were surprisingly reliable. Both were fuel-injected, which eliminated the trials of keeping multiple carbs sorted, but we were also somehow spared the misery of dealing with the infamous Lucas gremlins. Later in life, a friend owned a high-mileage late Series II XJ6, again surprisingly reliable. Thus began my love for the XJs, one which remains admiration from afar.
My dad bought a used '74 XJ12 L in the mid-'80s when I was young and it unfortunately lived up to the common perception of horrid reliability. The car liked to overheat on warm days, oil leaks caused it to smoke, the transmission went out, there were starting issues... the troubles went on and on. When it worked, it was superb... mostly it didn't and was a nightmare. Ultimately, it was left to rot for a decade or so before selling for parts to another owner.