I wouldn't worry about the pitman arm.
I'll second that.
Saginaw steering gear boxes for the most part all use the same tapered spline.
There are hundreds of different pitman arms to choose from, and as 6cyl said likely the original fits (I haven't checked).
I do know that I used an IH Scout II steering box on my truck, and used the shorter pitman arm form a Chevy Astro van....
So if you need a longer one I'm 100% sure you can use one form an IH Scout II, and there are other choices out there as well.