A third member out of a 1955 through 1962 Chev or GMC 1/2 ton will bolt into your housing. The torque tube was used as a torque arm and the axle housing is hinged at the springs. You will have to change spring seats, weld what you have tight or add a torque arm.
This is exactly what I did when installing my T5. We cut off the pivoting spring seats and welded on some solid spring seats from Classic Parts (~$20). The 3.90 ratio works well with my T5 and tire setup.
