Firstly, what do you mean by “in stock”? By the normal usage of that term the USN had only 3 of these barrels in warehouses after 1941.
Why? Because in 1920-24 20 barrels were passed to the US Army who reconfigured them for use in coastal gun batteries (5 x 2 gun batteries + spare barrels). Then in 1941, after the Iowa fiasco, the remainder, bar 3, (48 barrels) were similarly repurposed. At that point the US Army were proposing 27 x 2 gun batteries. 21 of these battery sites were constructed to take them but not all were armed. In 1945 the US Army still had 40 in active gun batteries around the US and Panama and possibly also Panama.
So the US Army has maybe 28 barrels “in stock”.
So problem 1. The USN, responsible for ship construction / refitting for USN and foreign ships, has to persuade the US Army to give a bare minimum of 3 + spares of these weapons up to add to the USN stock to even begin to think about re-equipping Jean Bart. At what point from early 1943 does the US feel safe enough that it doesn’t need to rely on expansion of its coast defences? AIUI that didn’t happen until late 1943.
Then we come to Jean Bart herself. The French first asked to US to assist in her completion as designed on 15 April 1943 and this was rejected on 1 May. Plans were revisited and a compromise with one turret with French guns plus a heavy AA battery of US weapons proposed. Again rejected 18 Aug 1943. There was then an appeal direct to Admiral King on 8 Dec 1943. In March 1944 the French were advised that the USN was unwilling to divert resources to the ship.
No doubt the experience refitting Richelieu between Feb and Sept 1943 helped inform this process. Ultimately the US refused to refit anything beyond Richelieu, the 3 Montcalm class cruisers, Emile Berlin and the 4 Le Fantasque class. Even then they refused to give them the then latest radar outfits.
Lest you think this was a dislike of anything French, proposed reconstructions of HMS Nelson and Rodney were also being rejected around this time.
So your proposal to fit these new 16”/50 turrets (if available) and guns involves far more extensive work than any of the proposals in real life. The whole forward part of the ship would need to be redesigned, torn apart and reconstructed to take the stresses and strains of the new armament. It is not just a case of slipping the new barbette inside the old. All the ammunition handling would have to be rebuilt as well from the bottom of the ship upwards. Then there are practical problems of things like metric v US measurements to be overcome in everything that goes back into the ship.
So even under the best of conditions such a conversion can’t start before late 1943. Time involved? At least 1 year. Look at the time that the reconstruction of Tennessee (8 months), California (15 months) and West Virginia (14 months) to get some idea of time involved. And that of Richelieu (7 months). OK less battle damage to fix but more reconstruction around the turrets. And if the turrets are not available then they may take longer to build than the reconstruction of the ship itself.
And all to what end? By the end of 1944 the Allies have plenty of Battleships in service and the Axis an ever diminishing number. Tirpitz has gone. Musashi has gone. The Italian fleet has surrendered. The RN has been decommissioning its older unmodernised ships for a year.
So for a whole variety of reason, political, practical and resources wise, this is a crazy idea that could never, ever get off the ground.
