I'm sorry but that's a horrible idea... can you imagine sitting in a claustrophobic space like that, especially near the back... only to view a screen that can't be any wider then what you see there?
You'd get a handful of lookie-loos and virtually no repeat business.
The bus should have nice 2:35 or 16:9 "screen" sections on the sides... and offer a drive-out theater. "We bring the movies to you"... then set up a nice outdoor theater experience, where instead of 22 people in a can, 66 people could spread out... and most importantly, the screen could be 20' wide, instead of 7' wide.