Have you changed out the shifting arm, so that it will work with the aftermarket shifters? If you have converted it over to a full manual valve body or trans brake, you can also get rid of the detent lever which takes up some room. My cars have custom headers, so I can't help you on your question, but with the after market shift lever and removal of the detent arm it should take up about the same space as the TH350.
Bill Mitchell