That's not necessarily correct... the profile is a percentage of the tyre width, so if you went for a 265 on the rear, you'd be right and want a 35 profile (to keep rolling diameter similar to std)... However a 265 won't fit on the front of a R33 gtst, so you'd need a 235 width tyre with a 40 profile for the front... they're the most common 18" tyre sizes for a 33gtst.
Something to consider also is the relative higher expense of an 18" tyre over a 17".... in your case I would stick with a 17 for overall looks and maintaining some sidewall height for compliance.
cheers