Here's a pic of your typical LPG/Gasoline tank setup, under the floor. Note that the tanks are designed to be stronger than the car chassis - so they will deform the chassis around themselves in a crash, rather than explode.
And a typical 6-cylinder setup on a jeep - you can see the injectors and the black propane hoses running to the manifold.
Here's the kit I used - http://www.tinleytech.co.uk/acatalog..._End_Only.html
(6 cylinder version)
Best bit is, all you need is a laptop and you can map the fuel to your heart's content. Now I want to advance the spark, but the propane ECU doesn't control that, the PCM does - so I have 112 octane fuel and standard ignition timing!