63-66 VS 67-76 A-Body Torsion bar crossmember

Does anyone know, what is the difference, between a 1963-66 VS 1967-76 A-body torsion bar crossmember? is it length? Torsion bar location?

I can buy a reproduction for the 1967-76 but not anything prior. I am wondering how much cutting and fabricating it might take to make the later model fit in the early model A-body.