I plan on cutting & opening 7' tall x 8' wide in an existing cinder block wall 8"x8"x16" that was laid with a stack bond (joint on joint). How do I insert an iron lintel 1/2" thick x 4" wide x 8'6" length? The length of the lintel is planned to go beyond the width 3" on each side. Is that enough and is 1/2" the correct thickness? The wall is 12' tall.

Thank you.