It's internals being that of a JGTC cars doesn't really matter, the McLaren F1's faster than a JGTC car anyway. I read the Z tune had 500Bhp, which is a far cry from 750WHP. The bottom line is, it's not enough to reach 240Mph without really pumping the power up 850Bhp won't do, it's too heavy and not aerodynamic enough, your only chance it to reall pump up the power, and if you do that you'll have to replace pretty much every component in the engine and gearbox. Even if you did manage all that, you've still only got a Skyline compared to the other blokes F1, I know which on eI'd prefer, and if I was in a position to buy an F1 ie more money than I can spend, I would buy the F1 over ANY modded Skyline.
EDIT: Yep the Z Tune is 500Bhp, nothing like the 750 you mentioned, I have no idea where you got that figure from, but all the sites I've checked have 500. And for a Skyline it's not cheap, $200,000 I could have bout the TVR Cerbera Speed 12 for around that ammount, now what seems value for money. Theres nothing wrong with liking modded cars, but it's stupid to say they are better that stock cars, it's an opinion. I like modded cars myself, but I don't have one, neither will I have one for a good few years, eevn then I'll only do underbody mods, I don't like bodykits. Performance wise, I don't think you understand quite how hard it is to make a 240Mph car that doesn't blow up before it gets there, no 900Bhp Skyline has ever done it, no 1000Bhp Skyline has ever done it, infact, I don't think ANY Skyline has ever done it. So no you won't be able to spend a relatively small ammount of cash on a Skyline and make it faster than an F1.