^^ I think only the Ultra Racing brace reduces ground clearance, all the nismo brace don't do this, if you have a look at both picture on the nismo site, the lowest point if either the gearbox mount or exhaust
EDIT: The picture posted by Sonicz from the r33-gtr.com website is the stock brace that comes in s15s rear boot, just find someone wrecking s15s and you can easily score them for next to nothing