Howdy, I have both an R200 and an R180, the 180 is a 3.36 and the 200 is a 3.55. Last time I tried to switch the rear end over to a 200 I had problems from He** and gave up, putting another 180 in to get me by. I did change mustache bars but nothing wanted to line up with the diffy mount bar. The victim is a '73. Two questions: 1; Do you need to change the diffy mount bar, originally I was told no, but now I wonder. And 2; Is the change over worth it? Stock motor, 4-speed tranny, good suspension. (Springs, bars, blah, blah, blah) Want a 3.90 but still looking. Is there a noticable diff between the 3.36 and the 3.55? Thanks for the info. Have a heckuva weekend... (And ok, three questions.)