Fire is not an issue. Plastic tanks are universally fitted to cars and almost universally to new boats and meet all fire regulations. Tek Tanks can make you a tank which can be mounted in exactly the way you describe. I have had two holding tanks made of similar capacity to your fuel tank that are bolted to bulkheads without any additional support. One of the advantages of a custom made tank is the ability to design it to fit your requirements.
That is exactly what my holding tanks look like. The welded Tek Tank types are much more substantial than the cheap rotationaly moulded ones which usually sit on a platform and held down by straps. Suggest you get a quote - in my experience they are competitive with a properly made and welded stainless tank, but obviously more expensive than a mild steel one. Always a bit wary of stainless anyway as it is difficult to get sounds welds.
The engineers who fitted my new TekTanks plastic tank were adamant that the two hardwood runners the old stainless tank had sat on were not good enough and would wear through a plastic tank eventually with the engine vibration. They suggested a flat base be glassed in which I did have done.