I myself am thinking of using a saginaw 4-speed with a sbc. I have one in a cosworth vega right now. The car is only a four cylinder as of now. Does not have near the horsepower that you are dealing with, but I have a 3.73 posi rearend as well and it makes for quite a fun car to drive. The only problems I have had was the speedometer cable gear was broken when I bought the car. I drive it fairly aggressively, but once again I don't have the horsepower your thinking of. I have asked around a lot to see how strong they are and I always find the majority say they will hold up to a sbc long as you don't go crazy with the horsepower and driving both. Does not sound like you are going too crazy with either. Good luck to you.