You have to try to get the engine to run in the 14000 - 15000 rpm range. This is where it develops its optimum power output.

The 10x9 and 10x8 props pose too much load on the engine to achieve this.

First, pipe the engine (although it will ruin the looks of your Stang). A standard muffler will hold you back.

Try 9x8 and 9x7 props.

The others are right though, that .46 FX will never make your Mustang go ballistic.

If you want real speed, try one of the common speed designs: Magnum, Dust, Whip, ...