You are not bursting my bubble. Lol
I had rebuilt over 100 Mustang front ends while I lived in the US ( have done 8 since returning to Oz), only 2 in the States required steering boxes & they were easy to find.
Yes some parts are non existant at the moment, but if there is demand, it will be reproduced. In the early 1990's you could not buy much "decent" repro parts for Mustangs, now most parts are available.
Good example is now you can buy cowls for 67-8"s that are an exact copy of the original. Most mechanical parts are covered as a majority of mechanical parts fitted more than 1 car.
I spent a lot of time, when I moved to California, networking with Mustang wreckers & other parts vendors. Now when I want something it is only a phone call away ( well also an airfreight !)
I can still get plenty of good used boxes in the USA, so a good used pitman arm is no problem. I would assume the pitman arm you speak of is also used on other models of Fords, homework for me to research!