OK Ive done this to my 63 Merc, my cousins 65 El Camino, buddies 57 Plymouth... Back in the day the way it was done could cause some problems. Now a days you buy a control box, a coil (2 for dual exhaust),and spark plug or 2. It doesnt work with cats, as they will not let raw fuel out the pipes.
Its wired so you rev the motor, press the button, and this control box cuts power to the engines coil and sends it to the coil or 2 you have set up for the flamethrowers. You drill a hole in your exhaust 8-12" from the tips and weld in a bung and screw in the plugs. So your engine doesnt get spark so the raw air/fuel go's through the exhaust and the plugs by the tips ignites and you have flamethrowers. Its pretty cool while driving cuz you can do a longer burn than when you are stopped and in nuetral. My cousin built an exhaust tip on his el camino to shoot the flames up at like a 45 degree angle. My 63 Merc had turndowns so it would blast the road and the fire would kinda roll behind the car. Never hurt anything. Was kind of a fun novelty thing for car shows and cruises.