Hey guys, just want to find out if there is a difference between those two. I am doing a AOD to 5-speed conversion and I ran into a problem. I have a bell housing that will fit the 3350, however, I just got a call from the person who is doing a swap for me and I was told that the bell housing will not fit. So I want to know...is there a difference between the tremec 3550 and the 3550 HD in the bell housings? If so what is the right bell housing to get for the 3550 HD? I look everywhere I and thought that all the tremec's used the same bell housings...can someone please help me out? I need to get the bell housing by tomarrow!!!