You need to remove the sterndrive to reinstall the bellows. It's also a little easier if you remove the hinge pins so that the gimbal housing floats rather than just pivots. But you need a special tool to remove the hinge pins. I gave mine away last year.
When you reinstall the bellows, ensure that the grounding clips are in place. The ground clips are small stainless steel pieces that attach to the end of the bellows, like a paper clip. The clip ensures that the hose clamp is grounded so that it does not corrode. Also you need to use a good amount of bellows adhesive. Coat both mating surfaces, let dry, then assemble. Easier if you have small hands and can reach inside the bellows, otherwise, consider investing in the bellows expanding tool.
Best regards,
Frank