Maybe that 73 swing pan is not so bad if it is in really good shape. All the parts are available for conversion and you can reuse the short torsion bars and swing spring plates ( by cutting). If the AS trans is attached to the pan it is dual side cover 3.88 and it can easily be converted to IRS. If you did all the prep/assembly work, welding costs would not be that much. I thought about that with my 73 and the big issue was likely having to detach the body from the pan to weld in the trailing arm brackets. Not an issue with just the pan.