You will have to find your own top speed and ride height. There is no conversion from GT3/GT4 to GT5 for ride height that I am aware of. I just try and keep the differences the same. eg. -10 front/-5 back for a ride height of 55/60 in the GTVAULT tunes.
If you look at the Zardoz tune with the most votes, the toe value for the rear is -1. Try +0.20 rear toe in GT5.