Flexible diesel tank, bladder tank

[Nyaminyami]

I have a 33ft Birchwood motor cruiser which had a 1400 litre stainless steel diesel tank which took up a heap of space in the aft cabin.
I am reconfiguring the cabin by cutting the fuel tank down in height to give me about 500 litres capacity.

I am pursuing the idea of installing flexible fuel tank/bladder tanks (probably two, of equal capacity) in the reduced-size stainless steel tank, using it as a 'bund' to support the flexible tanks.
I believe this would achieve several advantages but would like to get some advice from anyone who has done this before or otherwise has some practical suggestions or points of caution.

Thanks in advance.