Piecing Together a Power Steering System

In case you don't know, one thing that may come into play is your pitman arm from your 1968 is probably different than the one needed for the 1973 steering gear. The sector shaft to which it attaches is slightly larger than the one your 1968 gear. Also, the pitman arm stud that attaches to the center link is oriented differently in that it points down, not up. That's assuming both are the originals. All of this could require swapping the pitman arm, idler arm, and center link to match. There may be a large sector shaft pitman arm in the 1968 configuration. That would take further investigation or someone with more knowledge than me chiming in.

Also, does the engine have a pulley with a groove for a power steering belt? It could have been swapped out.

The T in TRW stands for Thompson. Thompson made auto parts for 50 years before RW came on board. It's possible the design of the pump predates the merger. Regardless, Thompson and TRW are interchangeable in this instance.