Quoting from the SA Design book "High Performance Ford Engine Parts Interchange" George Reid says, "The earliest 221/260 blocks employ engine mount bolt holes that are six inches apart, identifiable at a glance by the use of two freeze plugs. When the bolt holes were widened to seven inches as a means to improving noise, vibration, harshness and structural integrity, three freeze plugs were used. This change was adopted early in production in the 1963 model year. The 221/260 blocks should not be bored to 4.00-inches (289ci) because the risk of water jacket violation is very high."
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