I don't care what you do, unless you drown it out with pure audio volume, you'll always be able to hear your engine and exhaust to some degree.

As far as bang for your buck goes, I think buying a large amount of a decent product is the best way to go. Cover as much surface area as possible, especially in the big echo chamber behind the doors.

Most of what is marketed as "sound damping" material does most of its work by reducing vibrations, so covering all of the large flat surfaces in the rear is the first place I started. The brown bread reduces vibrations, and I've got a foam/rubber based sound barrier product from the same manufacturer as a second layer over the floor in the rear to help reduce the sound from the road and my PV muffler.

I've got a very loud system. On full blast, standing outside with the doors closed, you can just barely make out the highs and lows, but it's not even loud enough to hear the vocals in most cases. Open that door and you can probably hear it from 500 feet away. Regardless, I still hear plenty of engine and exhaust noise unless I turn it way up. Remember, the glass windshield and windows will always allow some of the audio waves to pass through.

Oh, and BQuiet has a completely different, high-temp product designed for use under the hood.