I'm not sure we can point to one FM station that used the "Stereo Rock" syndicated service, and say that one helped kill AM Top 40. Stereo Rock was on the air in quite a few cities, where an owner could automate it and put it on an FM station, while the co-owned AM station was fully staffed. I remember Stereo Rock on the air on WBEN-FM Buffalo (before those call letters were used in Philadelphia) and WGFM Schenectady (credited as the first full time FM stereo station). They both stayed automated after other FM stations in their cities had hired DJs. The ratings were good, so why pay a staff of DJs? Was Stereo Rock also used on WSTW in Wilmington DE?
And I don't think these stations overtook the AM Top 40 stations in their markets right away. In the 1970s, few cars had FM radios, so even if people started tuning them in at home, in their cars, they continued to hear AM Top 40 outlets. It was only when FM radios were in most cars that FM Top 40 stations pulled ahead of AM Top 40 outlets.
But the Stereo Rock stations really were great. No DJ talking over the intros, fewer commercials than on AM, and a voice always told us the last two songs after they were played. I suppose two songs were always recorded into the automation system back-to-back, so the voice could announce both together.