I was reading the service bulletins for the 350, and found this on setting carbs, includes idle speed of 1800. This is from service bulletin #BSG-0067:
7. IDLE SPEED ADJUSTMENTS
The idle speed sync is the most important adjustment for smooth idling. The correct idle speed is 1800 rpm. Adjust
both carburetors so that each side's exhaust sound is the same. On an improperly adjusted engine, one cylinder may
be running smoothly and the other will be firing only occasionally. When the idle sync is properly adjusted, the
exhaust sounds will be the same and there should be a smooth flow of exhaust from both mufflers. To verify that the
engine is properly adjusted, turn each idle screw alternately in a slight amount and RPM should immediately drop
200-300 RPM and pick up when the screw is again turned out. If idle speed is then too fast, adjust both screws an
equal amount to obtain the proper RPM.