"I'm a cheapskate.. My current XJ-S was a freebee from a junkyard.. (I just gave him the same weight of scrap) however they can be purchased really reasonable especially if not running.. (don't worry about that) an upper purchase price of $1000 and personally It would need to be really great to pay that much..
The great thing is a V12 is just three 4cylinders or 1&1/2 V8's Nothing at all exotic. except it's so darn well built.. (The important stuff not the luxury stuff). The best ones are made between 1975& 1980 but they are rather thin on the ground.. less than 15,000 made and sold world wide.. compared to the later HE V12's which they made over 77,000. California is where you want to go shopping. Rust free and the expense of meeting pollution laws turns some really low mileage ones into junk.. Parts wise sedan parts are almost all a bolt in and they made a whole pot full of those. So while a V12 sounds rare and exotic the parts are anything but.
As a combination use the early cylinder heads (they bolt right on) on a 93 or later 6.0 Gotta use different pistons though.
It will be relatively cheap to make 400 horsepower with that package and 700+ horsepower is available if you get greedy..
Best performance improvement? Gut the car.. stock it weighs 4200+ pounds You can get it down to about 3200 pounds easily enough. Next replace the automatic with a manual (4-5-6 speed) and while you're at it go to Corvette swap meets and buy some big rims. Stock the Jag has 15x6. You should be able with a minimum amount of work use a 18x8 and with more work use a 18x10 Just make sure the windows in the wheel are as big as possible.. (brake cooling)
A 400 horsepower Jaguar with a 5 speed, at 3200 pounds& 18x8 inch rims will be pretty darn fast and you should have less than $5000 in it..
Plus you just have to hear that V12 scream when it's unmuffled.. Oh and the idle will confuse you. Listening to it you'll swear it's at 1500rpm but the tach will be around 600 rpm.. That's because unlike a V8 it doesn't have cylinders firing on top of cylinders (which gives it the Burble) each firing pulse is 30 degree apart from the next.. "
"They only made 1275 in 1975 (that's what I have) Corvette uses 4&3/4 inch bolt pattern same as Camaro and a whole bunch of GM cars.. (plus big BMW's) Transmissions while not exactly bolt in are pretty darn easy.. depending on the year engine block You can use a Chevy scattershield Drilled out to the Jaguar bolt pattern. (The alignment bosses are Chevy size) and then after you bore out a Jaguar thrust bearing to the pilot hole size double check that it's not too far in. You might need to space it back a bit.. You will need a aluminum flywheel and pressure plate.. Just buy the clutch plate to fit the splines on the transmission..
Suspension is relatively easy.. If you've got it down to around 3200 pounds stock will be OK once you replace the rubber bushings with urethane.. (there is a whole lot more you can do but let's not overwhelm you)..
The brakes,well if in good shape are OK.. However Wilwood makes racing calipers that will bolt right on.. If you want to you can go crazy with brakes.. Right up to state of the art NASCAR brakes.. but the cheapest Wilwood brakes only weigh a few pounds compared to the 930 pounds the stock Iron ones seem to weigh..
There are tricks to this but just ask I'll walk you through them.. "
This was what was shared with me