Hmmm... the J66 is an auto, with I assume a moderate lens, and I think a different size roll-film to that of the normal conversion subjects. The best hackable roll-film Polaroids are considered the 110, 110A, 110B, and 120 - or similar list anyway. This is because of their super sharp, manual lenses and close proximity to pack-film size.
Still, there should be no real reason why you could not attempt to add a pack-film back to a J66. It might get messy though. I've not seen it done before, but there are plenty of references available - Alta Vista `Pathfinder', `110`, `pack film` and `conversion`, and you should turn up a few options. The trick will be to compare your case attributes with the usual conversion subjects... Good luck! Skj.