The GT option would be styled steel wheels (recently in a discussion elsewhere as to if they were chrome, argent or both were available.. have not nutted that one out), GT hubcaps, GT pop open gas cap, C stripe, dual exaust w/quad tips (requiring GT valence), fog lights, special handling package (suspension and steering upgrade), and GT badges on fenders.
Power brakes were required with big blocks, a separate option, but woud be on all '68 FE GT's. Big blocks also got a Chrome dress up kit under the hood, valve covers, air cleaner lid, radiator cap, and seen some debate on if the master cylinder was also chrome.