I remember back around '75 or so, some local guys built a straight six Chevelle with a Glide. I think it was either a '66 or '67. They were still running off national records at our track, I think. Anyhow, somebody running that same combo, bombed the record at some big race. So after that, the local Chevelle lost its advantage. So they quit racing it shortly.
It may have been a HP hit, instead of the lower record, that made the Chevelle uncompetitive. Who knows exactly when NHRA switched from records to the index system ?
I've read that it was started in 1975. But I don't know if that included the entire year or if it was switched over sometime during the season.