re: How did the 12 SEC teams agree on these East & West permanent rivals for over a decade?

Our past history with Ole Miss and LSU was mostly forgotten by the time we joined the SEC. We hadn't played LSU since 1966 and Ole Miss wasn't much of a threat after that series was resumed in the early 80's.

Tennessee was the only SEC-e border state and it just made sense that we'd play them. As mentioned, we played SC because we were both new.