The problem isn't so much the heat generation but the ignition timing, being of a higher octane value the LPG has a longer burn cycle than petrol and therefore needs advanced timing, unless it is advanced it then affects the valve seats as the gasses are still burning as they are being evacuated through the exhaust valves thus burning the seats away. IWEMA technologies make ignition advance modules that enable you to switch between the two fuels and run seamlessly, there are other manufacturers that do similar but I've used these ones and can recommend them. The 100% solution is to run an EFI system for both fuels through MS with the added benefit of being able to select between two purpose designed ignition maps that would be selected automatically when switching fuels.
Sorry, getting myself a bit of a semi with all this erotica, might have to do an engine swap in my Disco now to satisfy my craving........