If this the first time that you are building a multicopter, it is better that you start with a quad or a tri and learn first.
A tiltrotor is a very complex design. Stability is the biggest problem in such a design. Even though V22's are in operation, there are a number of precautions and restrictions in various flight regimes that the pilot must follow and take into account , a lot of research is on in this area but things have not been completely mastered/understood as far as the V22 is concerned.
Some of the problems are as follows:
While hovering, a bicopter has an inherent tendency to be unstable in the pitch axis, the fuselage starts oscillating about the axis joining the motors. Damping such oscillations require complex stabilization system(both mechanical and electronic). The real V22 has CCPM, variable pitch rotors, with gyroscopic stabilisation and pendulum damping.
The transition stage is the next major problem. Large forces arise due to gyroscopic precession of the rotors and the heavy engine and therefore have to be taken care of, the aircraft is most tentative during transition. Even the V22 has problems during transition and therefore, Bell Helicopters is working on a new design in which the engine doesn't tilt forward only the rotors do. It is called the BELL V280 Valor
http://bellv280.com/ .
Therefore a lot of research is required in this front. Moreover, you might have to write your own code, because as far as I know none of the quad control boards available have a firmware for a tiltrotor. Multiwii codes for a tricopter tiltrotor and a quad tiltrotor are availabe though.
IMHO one needs to do a solid couple or more months of research before one attempts to build this.