I agree. My only concern is if the crack opens into the garage wall cavity you could have a problem. Is there a way to make a tight mechanical connection between your new thimble and the masonry chimney? You don't want any backdraft to cause a fire in your wall.
One idea would be to seal the crack with Dap High Heat Mortar caulk #18854 available at www.doitbest.com. After doing this you may or may decide that it's unnecessary to install the new thimble.
Good luck with this and let me know if I can be of more help or if this didn't answer your question.
Thank you for using justanswer.