The upper control arm needs shortened about 10-20mm. It all depends on how much camber we want in the end.
The second problem is the bushing inside the UCA. It is difficult to find replacement bushings that are separate and the OEM one is a very unique bushing. It allows for all sorts of movement and is a seven-piece design bushing. The whole UCA on a G20 is about eight inches long.
The other thing that makes this more difficult is the angles that a G20 multi-link has in the UCA to maintain camber, toe, etc... and also function the way it was designed to give more camber upon more compression or more steering input. Those dimensions need to be kept in spec while shortening and that is the large problem.
You said you like a challenge and a lot of folks have said it cannot be done. I have seen some that are made from scratch out of tubular metal to accept OEM-like bushings and I have seen people just cut and re-weld the OEM arm to make it a small bit shorter. About 10mm is going to gain one whole degree of camber. Running around with -2.0 degree of camber will start to kill tires on the street-driving faster but handle much better in turns.