Trans Am racing and early 'Cudas

I decided this section was the best place to ask this question. While studying up on the classic (1966-1972) Trans Am racing series, I found some mention of a rule forbidding fastback cars prior to 1969; it was supposed to be a racing series for “sedans” only. However, I’ve seen various examples of ’66 and ’67 Plymouth Barracudas with the fastback body style being raced in Trans Am during those years. So, I’m either mistaken about the rule, or the Barracudas were not considered fastbacks during that time; but then I wonder why the fastback Mustangs (prior to 1969) were not raced then. Does anyone have any clarification on this matter?