The Oil Drum

"And I believe propane requires a new carburetor and not just adjustments to an existing one."

That's correct. It's usually referred to as a "dry gas" carburetor. Popular with 4X4 off-roaders as it allows for the truck to operate at all sorts of extreme angles without starving it of gas like a normal carb/float system would. Also doesn't spew gas all over the place if you accidently flip it.

"Propane through a modern, computer controlled car or truck, with not work without MAJOR factory level re-engineering."

Ehhh. Well the original (gasoline) fuel injection assembly would most likely be useless. I can't say for sure though as I'm not sure of their flow capability, or whether they'd freeze from the pressure release, and the pressure regulation system would surely be useless. However, beyond that, a place like ZDyne can take a car's computer and re-write the coding to change the fuel injection duration (the fuel injector is just an on-off solenoid that raises and lowers a pintle either allowing fuel to flow, or disallowing fuel to flow...so on any cycle the length of time the pintle stays open is what matters). By changing the fuel injection mapping you can control the mixture. Or, (likely easier) you could disable the original fuel injection system (perhaps just using switches to turn off the injectors themselves, meaning you could turn them back on) and put a dry gas carburetor in behind the throttle body - the rest of the system should act the same (delivering spark at the correct time, etc).

So while likely technically feasible...in practice, a bit rediculous and I doubt some poor farmer is going to want to pony up $2,000 bucks or something for some kit to switch to propane on a whim.