The tanks that Tec tancs can help with can be moulded to your hull shape.He will advise on rcd,as they supply most large boat manufacturers in U.K. I believe that you may also have a problem with this route, as it will need pressure testing and certification of compliance for rcd or mca codeing.
Flexible tanks may be an alternative,if so let me know as I may have a good contact for them.
I am assuming diesl is the requirement here.?I have no imformation on regs for petrol.