The tank is behind the cab half under the tray/bed and i still have my petrol tank in use and in std spot,you can get a little oil bottle that hooks into the system for the valves i havent worried about it ,my brothers 1fz-fe 80 series has had gas for years and when it did the headgasket ,we took the head the the head shop to get a clean etc and asked him and he said no damage what so ever to valves etc from gas or no damage probs in general
i service my motor every 5000km anyways so no difference ,as for power loss about 1% if any pretty much made no difference at all, unlike my older 3f cruiser 75 series gas power to petrol power was heaps different ,on petrol i run 95 or 98 if avl octain lpg is 108 octain and .all aussie 1fz's are injected no carby ones here