I wonder how much horsepower is required at 2500 rpm to comfortably cruise at 70 mph with the 2.71 rear end in a 4000 pound vehicle? is there a formula?
Yes there is a formula. I had to learn this for a Bonneville project I just worked on. You need to know the vehicles coefficient of drag, (Cd) the frontal area, rolling resistance on pavement, and total vehicle weight. Once you have those values the amount of horsepower needed to achieve a given speed can be calculated. The horsepower is independent of rpm and gear ratios. This is simply the power required to do the work involved.
For example, using approximated values, if your car has a Cd of around 0.44, and a frontal area of roughly 15 square feet, a weight of 4000 pounds, it will take 12.37 HP to overcome the air resistance, and 9 HP to overcome the rolling resistance. This means that your car needs approximately 21.4 HP to maintain 65 mph on level ground, in calm air. RPM and gear ratios only come into play if you can't make the required power.