Question about gearing for Top Speed

In my opinion, gear ratio is not a deciding factor, near as much as horsepower and drag, unless of course, the gears are too low.
Theoretically, a slant six with 5000 rpm capability, with 2.76 gears is capable of 150+ mph. Do you really think it will?
Theoretically, at advertised peak horsepower, (390 at 4900 rpm) at 4.10 geared A12 car wont break a hundred.
We all know thats not true.
My friend had a 305 camaro, with 2.29 gear in the back. That gear certainly limited top speed of that car, cause there was NO WAY that engine could pull that gear. Top speed would have been better with 3.55s.
Im guessing there is a rough guideline that land speed record racers are familiar with, but i dont know it. Even with the right gear, power and drag will determine how fast a car is.