My BF XR6 is fitted with lpg, its about a 50 litre usable tank, i get about 400 highway k's from the tank, costs $25 - $30 per 400 k's.
Im not an expert but just looking at the size of the tank in my boot i dont see how you could go much bigger,, unless you got a wagon or a ute.
just something to consider ie: you'de have to fill up in dalby so you could get out to site then back to dalby to fuel up again on the way home.
The dedicated factory lpg falcons would have a bigger capacity i'd assume.
i would also get the xt over the xr6 if i was buying solely for a long distance commuter, softer suspension, nicer ride.