Car appreciation... the aesthetics, the engineering, etc

That's a fucking crime right there, putting a builder's engine in a thoroughbred.

You could have a silky smooth V12, nigh on 300bhp in the standard 5.3 tune with potential for more than twice that.

Or if you want an easy modern swap how about the lovely supercharged 5 litre quad cam v8 with 510bhp and 625lbft out of the box?

Dirt cheap. Precisely. Fitting one to a Jaguar turns a thoroughbred into a mongrel.

The Jaguar V8s kick out a lot more than 400bhp in their supercharged variants.
http://www.jaguar.com/gb/en/r/r_specifications
The 5 litre supercharged v8 gives 510/625 except in the XKR-S where it gives 550/680.
Unsupercharged is 385 horses which is still more than a crate LS1

you fucking kidding me?!?
it ran an industrial 6 cyl tank engine designed during the depression.

LS1 swap is all sorts of win because:
-cheap, simple and light!
-stupid amount of aftermarket go fast bits
-bolt-in super easy, stock k-frame and powersteering
-little mod to driveshaft
-easy mix of ls1 power steering and a/c to jag parts
-ls1 rad straight in with twin fans (run cool all-day)
-with mafless tune and decent exhaust 300hp allday
-add baby cam and heads, 300+hp easy
-fuel consumption 12l/100k's, compared to stock 25l/100k's