Just let me clarify this: This wasn't meant to be a two-part story, but I guess life (and engineering problems) have a funny way of caring about your plans.
Step one: In order to have full adjustability on all three carbs I wanted to convert the center carb to an adjustable linkage as well. Unfortunately the fixed one is drilled slightly different and can't be fitted. So a new shaft had to be made: one 10mm ally bar
Make a nice snug fit 5mm center punch to mark the shaft precisely.
The punch marks are a lot more visible of the area has been marked with a felt pen.
I decided against the stock pin-setup for the throttle butterfly, which in turn meant making some room for the bolt head on the mill.
And as it didn't matter anymore anyway, I picked up some 15x15x2mm stainless angle bar and make a new set of rails for the carbs.
Final install and sync on the bike. (And yes, I am still waiting for the 6mm banjos, but other than that it's done.)
A first jetting at #20, #137.5 revealed it running rich on the top end, so after a slight rejet to #20, #125 that's definitely usable for the moment. Maybe a #22.5 pilot and half a turn less on the mix screw, but that's something for another day...