So I found an old dead keystone xr308 telephoto at a garage sale. The shutter is broken, the battery compartment is broken, and the viewfinder is broken. The only part in working condition is the film advance. I am considering converting it to pinhole. Has anyone here done something like this?
It's technically doable but don't expect good image quality. With such a small sensing area diffraction will eat you alive even if you get the pinhole small enough.
Yep, I agree -- even 35mm with a standard frame is near the minimum for good pinhole images. Even more so than with lenses, the bigger the film, the better the image will look, most of the time (there are images where the blur adds to the feeling, of course).