Ugh... sorry guys... It's been so busy over here!
VRT35 Nissan V12 DOHC 3499cc with 630PS out of a Nissan NP35 mid-engine race car.
VG30T/C Nissan V6 DOHC 2966cc with 680PS out of the Nissan R85V and R86V mid-engine race cars.
VRH30 Nissan V8 DOHC 2966cc with 750PS out of the R88C mid-engine race car.
VRH35/VRH35Z Nissan V8's DOHC 3496cc with 800PS out of the mid-engine R89C/R90CP/R92CP race cars.
VRH35L Nissan V8 DOHC 3495cc with 650PS out of the mid engine R390 GT1 race car.
VRH50A Nissan V8 DOHC 4997cc with 600PS out of the mid engine R391 race car.
Now try finding one of these engines and then try fitting it in your Z! BTW I think they made a limited number of R391 street cars for the public in Japan... but I may be wrong.