Under the tail shaft should be fine, might be a good idea to block it rather than just trust the jack as there is a definite pinch point for fingers.
Exhaust systems usually have a little flex in them because the are mounted in rubber mounts. you can possibly flex it with a big bar or a jack against them. I have no idea if that would gain you enough room to get in to the access hole, been about 55 years since i owned a 56. If that doesn't gain you enough you'll likely have to at least loosen/remove the pipe up to get access.
Good luck........
