[charlandk]

I have a roush 427 with similar hp to your motor. I am also using the t56. I am running a 3.91 rear end. I love the grunt it gives and it still will cruise on the highway in 6th at very low rpms. The only down side is that 1st gear comes and goes way too fast. The 1st gear is almost useless. That said, I would go with the 3.55 rear end.