A propane conversion is definitely not a modification that can easily be hidden from view. However, it can make sense as a reasonable modification in some cases.
Lower Cost Fuel
First off, the main reason to consider converting to propane is lower operating cost. If the price of propane motor fuel (sold by volume not weight) is significantly cheaper than gasoline (1/2 - 2/3), it can make a major reduction in the operating cost of a vehicle. For most hobby cars, operating cost is not an important consideration. However, if your vehicle uses so much fuel that you have to think twice before driving it to a local car show or cruise night, you may want to give propane serious consideration.
Generally, because propane only contains about 72.3% of the energy of gasoline (on a volume basis), carbureted propane-powered vehicles generally get around 75-80% of the vehicle's gasoline fuel economy. That is, if you get 20 mpg on gasoline you will get about 15-16 mpg on propane. If a litre (or gallon) propane costs 50% of the cost of gasoline, your 20 mpg vehicle will cost you as if it gets 30 to 32 mpg.
Let's say that you have a vintage fire truck that only gets a 10 miles per gallon. At $4.00/gallon, a 50 mile outing will cost you $20.00 On $2.00/gallon propane, your outing would have cost you $12.50 to $13.33. While this savings isn't that great for a vehicle that is rarely driven, it adds up quickly if you frequently drive your vehicle. You can easily hide the propane fuel tank inside the fire truck's water tank and fill up at bulk rates.
Readily Available Carburetors
The other reason to consider propane is that propane carburetors are readily available and reasonably priced, which is a consideration if you are having trouble finding a suitable OEM carburetor. Gasoline carburetors have been out of production for many years now and a vehicle hasn't been sold with a carburetor since about 1988. The carburetors that are in service are wearing out and the cost of repairing or replacing them can easily be much more than the price of a brand new propane carburetor. Coupled with the lower price of fuel, the switch to propane can easily mean the difference between a garage queen and a car that is driven regularly.
For those vehicles converted to propane, the engine should also be modified for this fuel. Propane burns a different speed than gasoline and the ignition advance should ideally be recurved to obtain the best power and fuel economy. The compression ratio can also be increased to about 10.5:1 to take advantage of propane's octane rating of 104. Because propane puts a higher demand on the ignition system, top quality ignition components should be used.
All propane systems must conform to the requirements of NFPA 58 in the USA and CSA B149.5 in Canada. Propane conversions are generally not recommended for the novice DIYer and, if you're not familiar with these systems, I recommend that all work be done and inspected by a trained mechanic.