Installing small block from the bottom

That is correct. The center link is too low. The center link goes below the right header, but above the left header. With the center link installed on the pitman arm you cannot put the left header in place. That is an ATSCO rebuilt gear. I checked their specs today, and they say the shaft is 1.25 inches. But the pitman arms listed for 73 Duster are 1.1 inches. That would explain why it only goes on that far. I can't find anybody else's specs for the size of the shaft on the gearbox and the hole in the pitman arm. Since the box came from Autozone and the pitman gear elsewhere, I am going to pick up their pitman arm yesterday, and when it won't fit we can have a little discussion with all the facts in hand. I notice that the gear they list for the 72 has a 1.126 shaft and I see no other differences. If I can't find any other data I may have to use a 72 box, and a 73 pitman. As for measuring, we are measuring with a caliper at the flats on the smallest opening on the pitman arm. The "correct" 73 pitman (Moog K7076) is supposed to be 1.1. As close as I can measure the TRW I have is 1.120. That make it about .125 (1/8 inch) smaller than the shaft.