4.56 Gears with a TH400
I personally would run no more then 4.10.
With that gear if revving out to 7k wll allow approx 126mph max speed. But if you have a GEN I SBC, I dont know what you plan on revving out too.
Based on the limited info, its going to be too much.
I had a TH350 w/ 3.08 gears in my Chevelle and it would run around 3k @ 75 w/ a stockish style stall.
That my exp.



