A 400 litre tank will actually only hold approx 360 litres,.. so 360 KG,.. the tank itself weighs in at over 25KG, and the tank fixing kit will bring you over 400KG total.
Now add the weight of your hose, reel, pump, battery, poles, trad gear, ladders (If you still carry them) and then the weight of yourself and possibly your passenger,.... and you'll be fairly close to the mark!
I'm in the same situation, a 600KG payload Pug Partner,... but I'm still going for a 400 Litre tank. I rarely have a passenger (I earn more when I work alone!) and I'll be leaving my ladders at home. I'll not be filling the tank to the top,.. just to be sure.
My reasoning is that sooner or later I'll be changing a van,.. and I'll go for 1 with at least an 800KG payload,.. so I might as well get the tank I'll need now and be done with it.