It cost like 2 grand for the kit I have heard. Road tax propane vs propane for other uses cost more. You can go longer on oil changes. You get less fuel mileage with it too. IMO you need a cheap sorce for propane to make it save you money.
I wouldn't do it. I'm rebuilding a '73 F-100 with a 390 FE that spent most of it's time running propane. The motor sustain some damage from the propane i.e. the rods were egg shaped because of the propane. Your friend will also need to put harden vavle seats in. My grandfather did this back in the day when propane was next to nothing.
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