If I did the following to my car, what would be the results? Ford Mustang GT Convertible?
Yes you could, but it would still get lower mileage.
Things to remember, filling a natural gas tank is very limited. You can set it up to fill it at home easy, but getting fuel on a road trip can be a problem. Also, the tanks are large pressure vessels, occupy a lot of space in the trunk. If the car is NEVER driven far from home, it works. Propane is a little more practical because places have propane for RVs, but some won't sell it as a motor fuel because of the taxes.
Last major issue, is if you live where it gets cold (scrape frost off the windshield), natural gas doesn't vaporize well until the car warms up.