Regarding return springs: For safety reasons (yours and the engine's) a spring should be attached to each carburetor throttle arm so that should the linkage to one or both carbs become detached, the carb will go back to idle. Otherwise the engine can suck them open to full throttle; I have seen this happen while I was standing next to the engine and could do something about it.. That (these) need not be the only spring(s). You'll still need one on the lower part of the bell crank, in my experience.
For rod-ends I have used Heim joints and threaded the ends of the rods which gives a nice, fine adjustment. You may be able to get left-hand threaded ones so you can make adjustments without disconnecting the linkage come tune-up/carb-sync time. A Heim-joint remedies the usual worn rods and holes in the throttle arms and bell-cranks. Washers can only make up for a little slop. Also try to keep spring tension to the minimum required or with multiple carbs your foot will get tired on a long trip.
