You say you have two of these tyres in your garage awaiting fitting.
Just make sure they are not the same as the front tyres. Your 18" rear wheel tyres, original fitments by Jaguar, should be P Zero 255/45 ZR18 Assymetrico - the wheel rim is wider on the rear wheels. They also have a higher load rating.
BTW, I would say that a lot of XK8/R drivers would not agree with HellCat's "best rubber known to man" statement.
I have used them for over 5 years on my XK8 until I changed to TOYO Proxis T1-R which are far superior in my experience. My car is in daily use with at least one Alpine tour every year, so the tyres are tested through the whole speed range of the car's capability.
You will also find that many have switched to Continental and Dunlop and much prefer them over the Pirelli.
My last 4 TOYOs, now just over one year old cost me £600 fitted and balanced, where the P-Zero would have cost me £228 per tyre at the time.
There also seems to be an ever-recurring delivery problem with Pirellis over the last few years.
each to his own dude lol
they have already been fitted thats was the price tag for fitting and all the other cost involved , even the nitrogen fill lol
imho they are still the best might take the beast down to silverstone lol