Even the M21 will not take to much beating on . Go with a B/W super T10 . This is the more stronger 4 speed . They were in 1976 to 1980 Z-28s and T/As . And Corvette's you sould be able to find one on your Local Craigs List . If you need any Ideas on the OLDs and Pontiac builds . Just ask I love to build them car's to . COOL