Big block A body pitman arm

I had installed big blocks using whatever pitman arm I had lying around (small block) and there seemed to be no fitment issues. My latest build will be using 73-76 steering components and I've already mocked them up to the engine and k-member. No issues at all. I have never really had anyone explain the difference in the big block factory pitman arms where it made sense as to why they used a different part.