There are 2 sizes for A-bodies.
The a-bods through model year 1972 use the smaller socket.
73-76 use the larger.
Personally I would not use an impact to install. There is a "suggestion" of a thread when inserted into the UCA but it is fragile, it is far from being a durable thread.
I remove them (or loosen them) and tighten them while the UCA is still on the car. The correct socket, a breaker bar, and my floor jack handle does it for me every time.
