I get about 250kms to a tank in my 351, full throttle whenever possible!
With the dead stock 6cyl I was driving around in for years I would get 300-350 driving in city traffic.
My current 6 cyl will be going to LPG later this year so i'm interested to see how that goes.
As far as mods go you'll need either unleaded heads (windsor only) or hardened valve seats installed. Stainless valves are recommended as is upping compression to around 10.5:1.
A decent exhaust and intake will help a lot too, the big bonus with straight LPG is you won't need to run any pollution gear which makes things a lot easier. If you do go straight LPG a recurved dizzy is essential.
Where could you find a windsor block? Try the docks... there are heaps being used as anchors.