Easiest way to tell is the size of hex nut under pitman arm. If a 1&1/8 socket fits, its the smaller shaft. If you need a 1&5/16 socket for that nut, its the larger shaft.
You can't use the larger shaft box in your early A. The pitman arm that fits both the larger shaft and your center link was never produced. The 67 through 72 owner would have to change pitman arm, center link, and idler arm to 73 up design. If my memory serves, the center link of first generation and second generation A-bodies are not interchangeable. If thats correct there is no way to use a larger shaft steering gear in a early A.
So with the correct steering gear... check that foot of gearbox and mounting pad of K member are clean and flat. Put some locktite on those 3 bolts and evenly torque them. Early Chryslers had issues there. The bolts would loosen, then the nuts welded inside the K would break loose.
Chrysler changed to bolts with special tighter fitting threads while they funded development of a thread locker product. Those 3 special bolts are offered in Parts For Sale section from time to time.