First, you need to verify the accuracy of your speedometer. Time yourself between mile markers on a highway, or use the GPS function that most smart phones have. 60 seconds for a mile is 60 MPH, etc. I'd expect your comfortable cruise speed for that setup should be somewhere in the 45-50 MPH range without overspeeding the engine. My 59 3800 flat bed with 18" dual rear wheels, a 235, a SM420 4 speed, and a 4.56 gear could cruise at 50-55 without any problems.
Jerry
