I would go with a Camaro/Firebird 1973 to 1980 10 bolt 8.5 . The Z28s had 342 Posi with the auto 373 with the 4 speed . The Firebird T/As had 323 Posi and some 1979/1980s had rear disc brakes with a 308 posi . A friend had a Firebird rear end in his 50 Chevy pick up . And they are not to hard to find . Also not to costly . The 1979 Monte rear end is the weak 7.5 ring gear with a 256/279 gear .