Veritech, your wife is indian right? I dont know whats with those indian chicks, they hate cars! But my gf (also indian) has come around slowly and now takes pictures of Zs with her phone.
a vg30dett will not fit in a z31 without huge modifications, i have only seen one in a z31 and it was a non turbo motor. Build the vg30et, they have been proven time and time again to be good for over 400 hp, jason84na2t is making almost 500 ft/lbs. take a look at his site redz31.com
im pretty sure bio-diesel and vegetable oil will only coagulate if it gets below 56 degrees. We run it in a yard dog and another truck and have been for awhile, no problems yet.
i'd say your car weighs in around 2700 + you, so Lets so atmost 3000 lbs, according to thins http://www.az-zbum.com/hp.calculator.shtml, you would run a 12.8, plug and play in the link there