Fitting tank to van

I'm just in the process of fitting my tank into my van. I have had a frame fabricated all I have got left to do is bolt it to the chassis, from what I can see most people just use 4 bolts with spreader plates underneath. Is this enough to secure it down? Any help is much appreciated.

Think about the weight your tank will be when it's full.  1,000lt is 1 metric ton.  Imagine a crash where you go from 50mph to rest in a fraction of a second - what's going to happen to that (say) 400kg+tank?  How thick is the floor of your van where you're going to fit your spreader plates, will it hold or will it rip up like a sheet of tissue paper and you get 400kg in the back of your head at 50mph?

If you're going to DIY it then look for the strongest points of the chassis and bolt right through.  That's the best you can do - if the chassis fails under the sudden stress then you're dead anyway, whether it's an all singing/dancing professional fit or a 'good as you can get it' DIY job.