Your challenge is to somehow make Johnny Cash President of the United States, with the POD being sometime in the 60s-70s (latest would likely be 1976).
60's-70's is far too late, you'd want something Pre-1954, and even then you have to have him desire holding any political office, let alone the Presidency. Representative? Maybe, but the Presidency may be a bridge to far for the Arkansawyer.