Too far aft!
I'm sorry I learned that, Tjar. It's absolutely the worst place for anything heavy to be located. As I told Dr McDaniel about his batteries, the boat will perform better from almost every effort to locate weight amidships.
If you haven't got a fuel tank at all right now, do consider installing one farther forward. A goal would be to add no weight if not between the after end of the v-berth and the bottom of the companionway ladder. If you can fit a fuel tank under the sole, there's your answer.
Polyethylene tanks have almost completely replaced metallic ones. Remember fiberglass tanks won't stand up to ethanol-blended fuels; but with diesel power you will probably never repower with gasoline, so even that is viable.
If one big tank won't fit, there are numerous benefits to having two or even three small ones (if they fit better).