What worries me Mark M is that you seem to be relying on the bulkhead taking the whole weight of the tank should you have a head on. I don't think it would. There is a huge amount of weight there, I don't know what size your tank is and it's entirely up to you what you do with it but I know I wouldn't want to sit in there or put one of my kids in the passenger seat. A flat tank might not be so bad but an upright would be worse.
Up to you mate.
What i have done is what so many others do plus more
I have used heavy duty rachet straps fixed to vans floor (eye rings fixed by renault)
150mm x 75mm wood bolted to chassis and fiberglassed in
1 builders band which is used to hold down your roof at home straped over tank and bolted to floor
one builders band straped around tank fixed though bulk head onto a steel plate
My friend is a engineer so he gave me the best bolts and steel plates