I never fitted the entire original slat floor with the plywood for a couple of reasons.
As mentioned by myself and others in this thread, this boat's one major weakness is the floor. One it's kind of thin and prone to little punctures when being dragged on the shoreline. Two, the seam where the floor attaches to the main body of the boat is susceptible to coming away if you place too much weight on it.
With that in mind, you really
don't want to put two whole flooring systems into the boat at the same time. And you don't need it either.
Apart from the weight issue, having
all the original slats in will force the plywood floor up into the inflated sides, whereas you merely want it to slide
under the inflated chambers for a good fit.
Also please keep in mind, although I do use one and recently two slats in the center of the original floor to support both ply sheets where they meet, those slats are cut down to 2/3 their original length. This is so they do not force the ply sheets upwards at the outer edges to avoid abrasion with the air chambers.
When it comes to mods, this boat's enemy is
weight and
abrasion. So think about those two things any time you come up with an idea.
Using only two cut down slats in the original floor at the center where the ply sheets meet, drastically reduces weight.
And once you fully inflate the keel, it pushes those two cut down slats up hard against the ply sheets to hold them in position and prevent movement that could result in tears, especially if a ply sheet develops a rough edge you don't immediately notice.
If you avoid the grief myself and others have inflicted upon ourselves with mad mods that didn't work you'll be fine.
Just get out on the water and don't overthink it. Experience trumps all. Hope this answers.