The O2 sensor goes on the exhaust pipe. Can go after the header. Must be a heated type to read correctly at idle. 3 wire or 4 wire. A NTK #21006 replacement is like $30. the gauge you got is fine. You just want to be real close to the 14.7:1 crossover point. Center light on the three light gauge. You can also read the O2 sensor output using a volt meter. Use your fingers over the idle air bleeds to shift the idle rich-lean and see the gauge react.
The exhaust must not leak to be accurate. a 1 wire O2 sensor (without heater) will not read accuratly at idle.
Buy some small precision numbered drill bits to measure the size of the air bleeds on the stock carb body.
or take it to a machinist and have him measure for you. A numbered drill bit set is like $10 not that hard.
Measure the drill bit diameter (or what ever else use use to gauge the idle air bleeds on the stock donor body with a dial caliper.
Try #70 or #71 primary jets in both carbs. (same ET, better MPG) The AFR gauge will show you the way.
The final primary idle air bleed should be a bit bigger than 1/16" .062" (.064 to .067") when you get it all done.
When evaluating a air bleed size with the AFR gauge be sure to check at idle and just off idle
Be sure the engine is fully warmed up and there are no carb or manifold vacuum leaks.
Measure pri and sec air bleeds in the stock 3310 750vs carb body and post the result.