[Tommy]

The air-fuel ratio problem does need to be solved. But here is some more to think about.
My car: 1135 Kg Cobra replica with 6.7 liter Ford Windsor engine, Holley carburetor, many modifications. RPM limited to 6000. 5-speed Richmond transmission with 0.77 overdrive.
On a fast track I hit 6000 RPM in 4th gear at 125 MPH, and shift to 5th gear at 5000 RPM. It accelerates well to 135 MPH but slower to 140 MPH. I think it would take 2.5 - 3 kilometers of straight road to reach 150 MPH. Aerodynamic drag on my Cobra gets very high above 130 MPH.

If shifting to 5th gear in your car drops the RPM enough to lower your horsepower to at or below the power needed for your speed, you will not accelerate. At any speed, the power available from your engine must exceed the drag on your car for it to accelerate faster. If you can safely go to a higher RPM in 4th gear before shifting to 5th, you might have enough engine RPM in 5th to accelerate more.