From what I understand, the CG of full-sized fixed-wing airplanes need to be forward of the wing's center of lift for stability. But the CG of a helicopter must be directly below the rotors since it doesn't have a horizontal stabilizer to counteract the pitching moment.
The c.g. limits change in forward and hover flight, so loading the aircraft takes special care. On the Bell XV-15, the range of c.g. was around 16 inches. For comparison, the internal cabin was 157 inches long. Based on the fuselage stations below and the image above, the c.g. limits were very close to the rotors in helicopter configuration.
For the control method in-flight, see: How do VTOL aircraft like the V-22 Osprey not tilt?
Commentary: how much forward ballast (see image below) an aircraft like the V-22 has -- I don't know (classified).
Maisel, Martin, D. C. Borgman, and D. D. Few. "Tilt rotor research aircraft familiarization document." NASA TN X-62 407 (1975).
If we take the horizontal stabilizer as part of the fixed "wing" arrangement in horizontal flight, then for trim the CG and center of lift must always align exactly (although the rotor/propeller thrust line may affect that slightly). Stability is a bit more complicated and is not relevant to your question.
The trick with a titrotor is to arrange it so that when the rotor tilts up to become a rotary wing, its center of lift still aligns with the CG.
You have to be real careful when loading up a tiltrotor like the V-22, to get the CG right in between the rotor hubs. Cyclic pitch control of the rotor can shift the center of lift a little, but not by very much.