If your focuser will not go in far enough with the eyepiece you wish to use, you will need one with a lower profile.

To get an image with anywhere near the telescope's resolution limit, you will need a guided equatorial mount for any exposure longer than about 1/15 second, because the image will drift about one arcsecond in that amount of time. A Dob would be limited to snapshots of bright objects, and Saturn almost surely is far too faint for such work. My rough and dirty estimate for your 100x attempt is about 1 second with ISO 400 film. An unguided exposure that long would cause a prohibitive amount of blurring.