You can get the crossmember you need from a Camaro or Nova restoration supplier such as Ricks First Gen. It may still be available from GM, but I'm not sure. It mounts to the same frame holes your current crossmember uses, but like Bob said, it's a totally different shape. It also locates the transmission mount holes about 1/2" farther to the right. This is due to the fact that the Camaro & Nova big-block engines were located farther to the right in the chassis than small-blocks were.
Go to http://rickscamaros.com/cgi-local/So...da8+1138125111
and click on "crossmember & rear mount" (lower left).