Looking from above and below, I can't see how tension adjustment is made. The compressor looks like it is mounted with four bolts to block. I can't tell if there are any slotted bolt holes.
You know I really like this car.
As for the factory service manual ..... well I really like the car.
Actually if it is anything like the shims on my 82 911SC, it shouldn't be very difficult. Of course with the 911, when you open the lid, it's staring you right in the face.