After WLAC went top 40 around 1972? (someone has the date) and you lived outside of WMAK's tight nighttime directional pattern, WLAC was the only local top 40 station.
It was October 1972 when WLAC switched from news/talk to top 40. Dick Downes (formerly of WMAK) kicked things off on a Sunday morning with "The Beat Goes On" by Sonny and Cher. Buried in my pile of stuff is the aircheck I made of that along with WKDA's final hours as a top 40 station before the change to country.
WLAC kept the CBS affiliation for several months after their format switch. Features like "Dear Abby" still aired along with the Doobie Brothers and other hits of that era. As I recall, the lineup was Dick Kent in AM drive, Dick Downes in middays and Dan Daly in PM drive. The religious block (World Tomorrow and Mull Singing Convention) brought the top 40 to a screeching halt at 7 PM followed by John R and "BluesRadio".
The block programming was slowly eased out and by 1978, WLAC was top 40 24 hours a day. They had about a 9-year run in the format until Sudbrink flipped it back to news/talk in 1981.
The mid-70's were a very good era for Nashville radio. WMAK was still going strong. With a mass-appeal approach, WVOL hit #1 in the Arbitron a few times and WSM was at the top of its game with a very tight hot AC format with "Your Announcer", Pat Sajak, doing a Dan Ingrameqsue presentation in PM drive. Folks driving in for the Opry must've been taken aback by what emanated from the Air Castle during the daytime hours.