Since I assume we're still talking about a breakout into the Atlantic while still docked in Brest, I suspect getting the turrets in from Germany is going to prove a mite difficult.
Actually, no. I'd forgotten what the thread was about and broken my golden rule about reading before replying. Though my reply to Cryhavoc101's is still valid for alternative histories for the Scharnhorst and Gneisenau.
Transporting the turrets to Brest although difficult is not impossible. If they had existed in 1941 the Germans could have put them on ships and sent them to Brest in the coastal convoys they were running anyway.
Yes the ships could have been sunk en route and if the British had known what the Germans were up to they would have been priority targets in the British campaign against Germany's coastal shipping.
Brest is one of the French Navy's biggest and therefore best equipped dockyards so it will have cranes capable changing the turrets because they would have needed facilities to do that for their own battleships.
Though when I was doing my research for the Axis France thread I found out that the French had tried to send some of their 15" guns to Casablanca so they could be fitted to the Jean Bart. The ship was sunk.