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Looks good to me, unless you've just poured a self leveler no floor is that level, especially when dealing with boards like that. Or chances are you'll scribe it now, but in a years time the boards will cup more and it'll be more gappy. Sometimes leaving a larger but consistent gap looks tider, or you can also expanding foam tape on a roll and poke it into the gap between plinth and floor, likely to have dust and dirt stick to it like a magnet though.
BrickMan-
Cheers, I'm going to leave the floor line as it is because, as you say that will likely change. I might fill along the underside of the carcasses as the sides of some of those protrude slightly below the bases (less than 1mm) so create some inconsistent shadow gaps if you look for them. The plinth is secured to the carcasses so less concerned about movement there. Hoping the sides are slightly easier as even though the walls are not perfectly plumb, they were recently plastered so should be straighter.
stevo_com
Did a terrible (IMO) job of scribing the plinth to the wonky as fuck floor. But it's painted and in now. Can still rip and replace if it bothers me, but ms_com is happy. Turns out the carcasses have through holes for dowels in the sides and top that provide the right amount of set back for the side and top pieces (the feet did the same for the plinth). So that's one less "how I am I going to make sure...?" quandry removed.