Mines fitted lengthways(500L upright)but up against the bulkhead so there's room down the sides. Personally I think it's in the perfect place and not overloading either axle. Diesel heater is bolted onto the back of my tank
Smaller car derived vans tend to be easier to fit a tank in. But the o/p needed a space between the bulkhead and the tank to store equipment in.
If your tank wasn't up against the bulkhead you won't have space for the hose reel at the back.
If you stand back and look at a Vivaro size van from the side with the sliding door open, you can visually see where the tank will sit between the front and rear axles, roughly in the middle.
If you look at my Peugeot Boxer van in the same way, an upright tank width ways up against the bulkhead isn't that far back from the front axle. You can immediately see that the majority of the weight of a full 650l tank of water will be over the front axle, totally overloading it.
My son has a 500l tank in his 2002 800lx Berlingo van. The tank is right up against his bulkhead which sits perfectly between his axles for best weight distribution.