If you dont want to plumb it in (and I'm guessing that you are avoiding this step because you want the ability to remove it at some stage?) then I would bolt some hardwood battens longitudionally onto the floor of your pick-up (for the tank to sit on). I'd then install some ring bolts / cargo rails and fasten the tank down with a couple of ratchet straps.
No welds to break and you can get rid of the tank when you dont need it.
For a more permanent arrangement though I would plumb it in - far less hassle for you once you are on the road.
One final thought 200 litres is a lot of weight for a hilux do you need that much? What range are you aiming for?
[This message has been edited by ctc (edited 21 July 2004).]