it seems that Amel installed autopilots directly on the quadrant which were based on the aft arm of the quadrant being extended aft. Older Super Maramus cannot extend the quadrant aft without making structural changes to the stringers, which is an unnecessary risk. So your solution of extending the forward arm of the quadrant with cosmetic changes only (not structural) is the way to go.
Yes it would be helpful if you measured the thickness of the arm extension that you welded onto the quadrant. I would be tempted to continue the quadrant extension with metal of the same thickness, or even to make a new one piece extended quadrant. The dimensions of the extension would be useful too, as well as a close up photo of the extension piece. No rush, it will be a while before I am allowed near the boat.
Super Maramu #151