What changed in 73?

There shouldn't be any problems with drivetrain if you use everything from one year or the other. Mixing and matching might encounter some minor differences. As mentioned, the front suspension changed along with the k-member and engine mounts. Everything in the rear axle and suspension should swap over. The wiring harnesses are different. Steering column is the same assuming same steering box, transmission, floor, column, etc. All the side glass (door and quarter) are different as are the regulators and hardware. The power steering gear changed from '72 to '73 model years. '73 started the large pitman shaft. You would want to match the box to the year suspension (and pitman arm) and k-member you decide to use. Otherwise, you're looking at getting some conversion parts or making modifications.