Hy 58GMCanuck, I found two different numbers for the extension housing seal, for a 58 standard three speed Napa part number ATP 12727 $2.99, for a 59-62 standard three speed Napa part number NOS 15041 $2.99, [my parts book only goes up to 62]. If it's a heavy duty three speed it will probably be a different part number, hope that helps.