Wot car U got?
My mate (let's call him dogdy dave) has a peugout 405 and he sometimes slides a 400L tank in the back with the seats down. Even he won't go more than 300 litres in it as the suspension starts rubbing!
I hate to think what would happen in a crash.
My honda estate (mondeo sort of size) has a 125L tank in the far back plus a 25L barrel too (and battery, 30m hose and pump). The tank is up against the wheel arches so can't go forwards. On the back seat, on a wooden platform secured by seatbelts live my empty trolley and shurflo back pack.
On the front passenger seat in a frame lives a 25L barrel and in the front footwell live two more. So I carry 240 litres spread evenly and well secured. It's not enough really and a van will arrive in time!
(May be a big volvo type estate and you could stretch to 300 litres max spread correctly in my opinion)