Just glancing at this... a long, long time ago (1978?) I handled what I think I can recall as being an F2 Photomic? Is this what this camera is? For my small hands back then, it must have been quite a haul-up (being confined to a wheelchair with distrophy at that time, since long, long recovered). But a few years later I was the owner of a Nikon F3, probably the next new kid on the block after the F2? Fascinating that the F2 workhorse is still in use and much loved by so many. I vaguely recognised it from the photo in the posts. I haven't seen one second hand for many years so I guess they are rare.
Your FM2 with the high shutter speed of 1/4000 does have a vertically travelling slit but since it has 1/200 (fm2) flash sync speed and that means the shutter curtain travels very fast and reducing the distortion you wanted to almost none.
Correct. You need a Graphic or Graflex with the slow traveling shutter curtain, use a low tension setting and the smallest slit.