I'm talking about the fullbacks, the 4312 that Pirlo's beard is unable to fullbacks for example against 433, because the back 4 is busy with the 3 strikers and when the fullbacks go forward the diamond (which is the most important part of the 4312) disappears. Cruyff explains this in a utube video
You are right that there isn't a natural tactical match-up between 4-3-1-2 and 4-3-3 because of 4-3-3 having two wide players on each side.
However, I'd suggest there are a number of options.
The classic option is for the central midfielder on the side of the diamond midfield to drift wide in order to meet the full-back. You then ask the trequartista to drop a little deeper in order to fill the gap left by the centre-midfielder drifting wide.
Another option is to ask one of the strikers to track back with the full-back. This depends on the type of striker as a player like Tevez could do it, but Llorente couldn't
How you are going to play against the 4-3-3 also depends on exactly what their players are like. Are both the wingers actual wingers or do they spend most of their time drifting inside, in which case the fullbacks in 4-3-1-2 can allow them to drift inside and leave a central midfield player to mark them, leaving the fullbacks to play against the opposition fullbacks.
Also, does the 4-3-3 team have an attacking fullback on both sides? Maybe one of the fullbacks can be ignored either because he doesn't attack or because he is not considered enough of an attacking threat to close him down (I recall Allegri making that call when Milan beat Juve 1-0 at San Siro. He decided it was okay to give Asamoah time and space on the ball because he didn't think Asamoah's crossing was a danger).
While there can be this issue of closing down wide players for the 4-3-1-2, there are questions about being outnumbered in the middle of the pitch for a 4-3-3 against a 4-3-1-2. How does the 4-3-3 adjust to stop the 4-3-1-2 controlling the game with the extra man in midfield? Perhaps they will have to make one of their wide players play narrower, which will then remove the problem for the 4-3-1-2 team of being outnumbered out wide.