With SX-70 you have reflex viewing, can do manual focus, and have primitive control over exposure. However, you will need an empty film pack to ensure the film sheet is in the proper plane and to provide power (the battery is in the pack). Also, you want to ensure it doesn't get ejected!
You may need a "fake" sheet so that the camera doesn't think the film count is at zero. Then put real film on top of that. Maybe trim back edge of film back a bit so the ejection arm doesn't push the film into the rollers after the photo.