Viewed as a head-on cross-section of mine, the interior flooring on the two gunnel sides begins to slope in at a gentle angle from around the point that the angle of the "V" itself starts, but not as acutely. It then flattens out after about a foot into the central five feet (or just under) of width, which a tape measure into the middle part of the cabin bilge tells me is about 10" higher than the deepest part of the "V."
I am not sure how well I have explained this, so if you cannot visualise it I may be able to post a sketch.
So basically 10" from the deepest part of the centre of the V is the lowest that you could have lain a reasonable width of flat floor in my boat whilst using unobtrusively sloped sides. Lose another 3-4" from that if you're not going to slope the outer edges.
Aside from one short side walkway next to the bed (which feels wide because of the open space next to you) my walkway is central. The walls of both sides are taken up with fittings-shelves, bench seat, cupboards, stove plinth, shower, bed etc, so your headroom is always in the highest point of the roof too.
I honestly don't think it would be possible to drop the main breadth of the floor any lower than the aforementioned 10" from the deepest part of the "V," due to the angle of the V itself-assuming the shape of your hull is pretty much identical to mine.
The main walkway through the middle of my boat varies in width due to the fittings either side, but it doesn't look as narrow as the aisle on that picture above-Say anything between just over 2ft at the very narrowest (again, with open headroom) to 4ft at the widest. Or maybe it's just the angle of your shot... Or the fact that the width does not deviate at all, making it look longer and narrower than it really is.
What do you want to know in the way of building cupboards etc? You basically just have to cut the side ply at the angle of your slope... The back walls of mine are the boat's old interior tongue and groove in the main part.
Ps., if you think this is fun, wait until you need to fit a shower with enough head room... I am 5'9" and it was still a challenge. And my Dad, who is 6'7" has only made it inside once in five years, funnily enough... How long is that boat, btw? It looks pretty small.