Doubling the rpm at 70 and projecting that you can attain 140 has one basic flaw.

At around 80 mph wind resistance, drag, becomes the greatest force to overcome, vs, rolling resistance and drivetrain friction. The problem is that you must have enough HP in the motor to push the car to 140. There are ways to figure this, but imagine an old VW bug doing 70 mph at 3000 rpm. If you spun the motor in top gear , no slippage , then you would expect to see 140 at 6000 rpm. Problem is that if the motor only makes 80 hp and it takes 120 hp to push it to 140 then you will hit a wall where the motor rpms do not rise and the car does not accelerate any more.

Now TBirds and the sibling cars are aerodynamic enough to acheive these higher speeds at lower drag values, but then you have the acceleration issue. As you approach the wall of HP vs Drag your acceleration increases at a decreasing rate. Basically, if you need 175 hp to push your car to 140 and you make 180 hp you need a REAL long road to get that high and you need to hit 140 at the peak power rpm of the motor. These motors make peak power at around 5300 rpm in mostly stock form.

You need to look at the dyno graph of a motor. Find the HP at the RPM you expect to be at when doing 140, IE 4000 rpm, and you can see you are maybe making 140HP. So, to attain the 140 with your motor at 175hp you need to be at say 5200 rpm. The only way to do that is to be in another gear or have a different rear end. Guess what that does, reduces the top speed. Doing a Bonneville run requires all this stuff to match up when shooting for records.

Now, lets say you have 400 hp to the rear wheels. The top speed has to be figured by finding out the equation of drag to determine the power required to accelerate up the range. Then you plot the HP curve of the motor against that and where they intersect is the top speed of the car theoretically. Hills and air density then start to become small multipliers in that area.