Glenn, my slim jim transmission is strong - not in need of rebuild. I putting a 3/4 race came in my 394, as well as an offenhauser dual quad intake, solid lifters, and having the heads polished/ported. My fear is that the slim jim will not perform as well with the additional horsepower and that's why I was thing TH400 transmission. Perhaps I'm wrong, perhaps the slim jim is just the ticket?