Well for the P/S I know the '77 unit I'm using is beefier than the '72 and uses different idler arm, '77 has pin for idler in the drag link while the '72 has pressed in bushing... I went with the later unit as a complete idler is readily available... AFAIK idler arms are year specific, whether M/S or P/S makes no difference...
Yes, they are different. The 1970-74 pitman arm has a hole in the pitman arm to accept a stud from the center link. 1975-1977 is just the opposite.