Thanks Ned, that makes sense about the film back.
I plugged in a 2.5" focal length to the pinhole calcuator, and it looks like it won't give me enough coverage. I increased it to 3.5" and here is what I got:
Focal lenght: 3.5"
pinhole diameter: .016" (.4mm)
film dimension 6.4
angle of view 84.9
Does all of that look good? I basically tried to get the coverage larger than the film dimension.