I understand you Pierre, but my point is that it doesn't matter where the tang is fixed inside the hull.

You can just assume that nothing is wrong with the final fixation from the tang somewhere in the hull. (otherwise it was broken there!)

So you make room above and just under the tangstrip as far you need the clearance for drilling a hole in the topend of this tangstrip and fixing a bolt through the tangstrip and the two new strips for lengtening the tang to its original size.
In this ""room-making"" you follow the shape of the tang; which I don't know either. It means that you have to cut (saw) anyway a groof in the deckside. Worst problem which you can encounter is when the tang goes vertically down. In that case you have to drill a hole from outside the hull in the tangstrip for the fixation of the strips.
You can then also consider the possibility of taping thread instead of bolting.

I agree it is then becoming a hell of a job; but I personally would take that risk before removing and fixing the whole deckside. Though I've done this once long time ago in the summer on the beach (somewhere deep South-France) on a dart-18.