Good morning Hugues,
I am not sure, but I believe the settee berth (banquette) had a small narrower cushion under the main cushion to level it out, possibly secured with velcro, but perhaps not needed, because where your feet go does not have much influencing it moving around. Or maybe left it alone for storage
of items like spare life jackets. I'd suggest that you measure, bearing in mind your requirements, and determine for yourself the sizes you will wish. Our cushions
, for instance are firm foam, 4" thick. They may feel too firm at first, but one gets used to them over time.
The condemned tank now represents waste space, unless you can figure out a way to re-commission it.
The space in the forepeak is normally used for storage
. What you store in there is up to you, it is a space that will accommodate longer items.
Maybe someone else out here can tell you where to look for the hull
Hope this is a little help.