I'm late to the party here, but there were two dual fuel cars in the family back in the mid 1970's - the days of oil embargoes and CA state subsidies for "clean air" stuff. One conversion was my sister's 1972(?) AMC Hornet 258 and the other was my dad's 1971 Newport with a 383 magnum. Both were done by the same outfit, and shared the same basic setup.

Here's what I recall:

Tank in the trunk, taking up much of the space in the Hornet, and fitting easily in the Newport.

Had a state sticker that exempted us from the state tax on propane (early tree-hugger "clean air" stuff). Got us a good price at the pump, but you had to fill up at a short list of propane stations. You could not buy from the fill-your-BBQ-tank deal behind the gasoline station.

Exemption from NOx retrofit requirements.

Delivered liquid from the tank to an underhood mixer. If I remember correctly, the mixer had engine coolant plumbed to it.

The system used the throat of the carburetor as the propane delivery device, with a large metal elbow in place of the air cleaner assembly.

To switch between fuels, the driver hit an electrical toggle switch (fuel cutoff valves) AND pushed or pulled a plunger (cabled to the air diverter).

Power was noticeably down when running on propane, and overall vehicle performance [no matter the fuel] was not helped by lugging around a big propane tank in the trunk. The hornet got sold with the equipment on it, but the Newport was traded in on my dad's next car, and the dealer absolutely required that the propane stuff be removed. I took it off for him, and had the use of the car for the weekend before turning it in on Monday. Boy am I sorry I didn't have a chance to drive the Newport longer - that car was a surprisingly fast car running on gasoline and not carrying around all the propane stuff.

On a dual fuel car running propane through the carb, it is ABSOLUTELY NECESSARY to run it on gasoline periodically. My dad was not as careful as he should have been on this point. One time, the car was run out of propane after not running on gas for a long time. I was tasked to drive it down to fill it with propane. It ran like crap, started smoking at the end of the trip, and died just as I pulled up beside the twenty foot tall tanks of propane.

Trivia:

a. A sixteen year old on adrenaline can single-handedly push a 1971 Newport about 50 feet, especially if it is on fire beside twenty foot tall tanks of propane.

b. There are a surprising number of fire extinguishers on hand at a propane station, and even the clerical help in the office are willing to wield them if it seems necessary.