I have my basic K jet 500sel running on dual fuel, petrol/propane. Its a pretty easy conversion if you go for the single point system.
You need to introduce the gas by means of a Mixer. This simply plums inline with the air inlet of your engine. The mixer is then attatched to a vapouriser/reducer, which converts the compressed liquid gas into low pressure vapour for introduction into the engine. This is plumbed into the tank, which I have located in my trunk.
On my car, I simply have a relay to turn off the fuel pump when the propane solenoid opens. I would imagine on an older EFI, you could disconnect the injectors by removing the common earth, so the computer is still running and controlling the idle etc, and the fuel pump is still running, to maintain pressure for when you next start the engine.

Hope this helps

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Ben

'85 500 SEL
'85 928S2
'88 560 SEC
'90 750i