Nitrous is basically oxygen. What I mean by that is oxygen is the important element when you talk about injecting nitrous into an engine. The nitrous gas breaks down during combustion and extra oxygen is added to the combustion process.
You need to add more fuel to match up with that oxygen. If you have less fuel than oxygen, you are lean, and bad things happen. That's why nitrous can cause meltdowns so fast, you are adding 200HP worth of oxygen instantly, and if their isn't 200HP worth of fuel at the same instant, you can have a BIG problem.
Normally gasoline is used as the extra fuel. This can create problems on certain cars. Most commonly the gasoline is injected in one spot in front of the throttle body. Not always, but this is the common way of doing all but the most extream nitrous setups. Some intake manifolds do a terrible job of dealing with fuel moving through them (like a Viper). The fuel can puddle in certain areas of the intake manifold. It can ignite, basically blowing the top of your motor to bits. Or it just doesn't flow the fuel properly, and you go lean in the cylinders. See paragraph 2 for what happens then.
Propane on the other hand, is a gas at atmospheric pressure and temperature. Which means as soon as it passes through a nozzle into the engine, it immediately turns into a gas, just like nitrous. It doesn't need help to atomize like gasoline. It doesn't puddle in the intake manifold like gas can. It also has added benefits of being a higher octane than gas, at about 105 octane. Someplace stated 140 octane and that's bull. Do some serious research on the subject and you will see propane isn't even close to that figure, but 105 is still much better than 92. You also don't need a to touch the stock fuel system. The propane doesn tax the injectors or fuel pump. It just needs to have compressed liquid propane in the tank and it will pump itself out. You use a pressure regulator and a set nozzle size and are guareenteed a linear flow of propane. Just use a nitrous solenoid to open and close the flow.