Proof?
While I agree that Johnson was a reprehensible individual, anything stated by his convicted swindler/liar associates like Billy Sol Estes and Bobby Baker is totally worthless. The problem of Johnson's associates is double-edged: these characters are capable of anything, but because they are so rotten you can't believe anything they said or any evidence derived from them.
It probably was a conspiracy. Johnson and Oswald were certainly capable of doing this type of crime. However except from some possibly planted (or not planted) evidence by the Dallas police and/or FBI there isn't much proof against Oswald, and I don't see any real proof that Johnson was involved.
I need legal solid hold-up-in-court proof, proof, proof, proof, proof, proof, proof, proof,proof, proof, proof, proof, proof, proof, proof, proof, proof, proof, proof.