Can the fuel tank not be moved?
Might seem a lot of agro but its normally only 2 straps to be unbolted once the filler necks off.
If you are welding the tanks brackets to the load area then you should place something fireproof between the tank and floor if its petrol, and have someone on fire watching duties armed with an extinguisher or a means of putting water where you need it, ie; hose.
As for the strength of a weld, I used to have to do test pieces on a regular basis which involved butt welding 2 steel strips with an inch of weld @6mm bead, these had to be tested to a ton, so if you are competent and have good gear then a welded joint is better than a bolted assembly as the bolt can pull out through the floor if it has not got a big enough mounting plate on the backside of the bracket.
Having said all that if I were putting a 400 litre tank in the back of a van I would probably go for the bolting method as it isn't going to permanently damage the floorpan apart from some holes, and I would make up some plates to go on the opposite side of the floorpan to give the mounting points some strength, you only find out how strong a mount is when its fully loaded and put under severe pressure, normally when some tw*t pulls out on you at the last minute, Andy