From what I read it was 16mm- shot on an Arri 16s which uses an electric motor (my foggy memory thought it was a Bolex 16mm, which does have a spring wound motor. I never read his book Rebel Without a Crew). 100 foot rolls- IIRC that’s 2 minutes 40 seconds @ 24fps.
I don’t recall any S8 features making their way to a theatrical release, but I may be wrong- I think 8mm or S8 blown up to a 40ft screen wouldn’t look that great. I could get carefully shot S8 K40 to look almost as good as 16mm- carefully shot 16mm or S16 (better!) can look good in a theater. Post synchronized sound works- the French and Italians were great at it- they shot most exteriors MOS (mit out sound, as the story goes).
Many years ago there was a guy who ran a S8 post production facility- I was working on a project with a friend- and there was a guy cutting a S8 feature at the same time. Boy, it was rough looking, but the look fit the story and it was a powerful film.