I be following the build up 67 mustang and the articles on add a 8.8  rear end from a fox body  
I would like to do the same to my 66 mustang I know the 67 are wider than the 66 will it work?  

Do not put a 8.8 in there. Go find yourself a 9 inch and do it properly. By the time you get done with all that you still have a weaker rear and reall y have not saved all that much . Heck for most street cars a 8 inch will be just fine and cheaper and dang near as strong .