As it was stated multiple times in other threads, the JGTC rules (at least for the 500 class) are very liberal. You can use nearly any engine you want (AFAIK the Toyotas had also other engines than in the production cars), you can mount it how you like (for the NSX it was vice versa compared to our cars), you can build a whole new suspension etc. - so the race cars have nearly nothing to do with the cars in the showroom.
That's also the problem for someone who wants to use a JGTC car for Le Mans or other european challenges: The rule books are too different.