As Ancelotti and Martino have already confirmed, tomorrow's game will be crucial in the race for the title and no experiments will be made. Real Madrid's lineup is perfectly clear, and Barça's plan is decided also, although I still doubt who their right winger will be (Neymar, Alexis or Pedro).
Martino's men have not been doing their best football lately, and results speak just by themselves. Their losses against Valencia, Real Sociedad and Valladolid have all followed a similar scheme. The 4-4-2 with a narrow diamond-like midfield has been an extraordinary weapon to beat Barça, mixing the accumulation of men in the center to complicate Barça's circulation and the hard work of the two strikers against Piqué and Mascherano, both in offensive and defensive phases.
Real Madrid's 4-3-3 can, in a way, mimic this behavior, just making Benzema take a step backwards to act as a mediapunta and seeing Ronaldo and Bale in more central lanes, occupying the space between Barça's fullback and centerback in both wings.
This way, Barça's starting is almost broken. As you know, the culés usually begin their possession in "lavolpiana", that is, the fullbacks advance, the centerbacks move to the wings and the center-midfield (Busquets) comes down to occupy the space between them and receive the ball. This is a natural way to start when you are pressed by two strikers, since you have three men occupying all the width of the field. However, when playing against a 4-3-3, you need an extra help, and Xavi will naturally provide it.
If the BBC is focused, and Benzema specifically since he will have to deal with Busquets, we will be able to prevent Barça from moving forward. A concrete job of Di María over Xavi seems essential also, and, if the Fideo can keep performing at the intensity he is showing in the last games, the Busquets-Xavi connection will be broken. Sergio, or even the centerbacks, will need to take risks with long balls and there Madrid's ability of getting the ball back and counterattacking can unbalance the game.
Unfortunately, things won't be that easy. Our opponents' talent is absolutely out of question, and, undoubtedly, they will be able to cross our first pressure line in certain occasions. The biggest danger there will obviously be Lionel Messi. To stop him, as we have done in previous confrontations, we need to keep our defensive line as far from Diego López as possible. If we do so, there will not be enough space for him to receive from tXavi or Cesc and turn around. Perhaps having the fullbacks move a little closer to the centerbacks can help in this setting, since Barça's wing men are not too strong when they can't occupy central positions, but we need to be careful in the right wing, were Carvajal and Modric will have to deal with Alba, Cesc, Iniesta and eventual appearances of Messi. Be ready to suffer there.
Let's take the ball and attack, now. Our low rhythm can, literally, exasperate Barça's midfielders and attackers. Other than Busquets, none of them excels in stealing the ball, and having them run around pursuing us can be a great tool to open lines.
We can win the game in our left wing. Marcelo, Di María and Ronaldo, together with Benzema's help there, will be way too much for Xavi and Alves, and even more if we are able to occupy Xavi's back, which he is quite used to leave undefended. This way, Piqué and Busquets will need to come and help there, and Bale, Carvajal and Modric will be waiting in the weak side to finalize. Aside of that, Barça's weakness when defending set pieces is manifest, and we might be able to take advantage of this at some point. Finally, if this does not work and we are in trouble in the last minutes, be ready for some Disco from Isco.