Rhys, I was researching an old Motorsport Magazine article from 1964 that very interestingly it covered a road test on a 1964 Monte Carlo Rallye Falcon and they stated in that article that the "fiberglass panels were wafer thin" so you are right they would have looked like the pacific ocean when looking down the side of a painted fiber panel. I have left some of that type of deflection in my build (it is fairly marginal though).
They also commented on how surprisingly well the American Falcon handled and how relatively light the manual steering was.