[mr48chev]

I'm thinking that the big difference in the front steer Firebird Subframes is the front end of the subframe where the bumper brackets mount. The earlier ones having bolt holes for the bumper brackets to bolt to and the later ones having the sockets in the end of the frame rails for the energy absorbing shocks to fit in and the four bolt tabs for them. I think steering box mount and A frame mounts are the same.

I'm not sure about the spindles and brake rotors but the 71/72 may be different than the 73 and up. The change over on Chevelle style (size) spindles and rotors being with 73 year model.

If your A arms and the rest of the suspension and steering is good you might be able to pick up a subframe that someone has stripped the suspension pieces off for a low cost and go from there.