DO it. Make sure to not park the car inside a garage or parking deck.
That is Obviously an Engine Fire, not an Exhaust fire...
If done right, the way the hot rod crowd does it, then you shouldn't have issues... Just make sure to only hook the propane bottle up at shows and not going down the road (or the drag strip, you could hook it up there for effect).
It is highly illegal to have a flame thrower kit installed. However if you go and buy a 2009 F350 Diesel, that hasn't been updated by FORD with the appropriate TSBs, then you will have a Truck with a factory flame thrower. They used to shoot 4' flames out the tail pipe because of an over fueling issue during regen and caused this effect.