1967 273 4bbl with Edelbrock 500 CFM Carb

Crew,
You can use any gear. The idea is that the engine needs to have the converter load on it.
The idle speed screw is on the main body, a single screw, on the driver's side, adjusted from the front of the carb. It just controls where the t/blades stop.
I am going to have to disagree with RRR about using a vacuum gauge. Not needed & you don't get the best result. The human ear is far more sensitive than a vac gauge. Screwing the mixture screws in leans the mixture & vice versa. Always aim for the leanest mixture screw setting that gives the highest/smoothest idle rpm. The highest rpm indicates that the engine is making the highest HP with amount of air & fuel being supplied by the mixture screws. If you turned the screws & rpm drops, engine is making less HP.
There is no particular idle rpm once you deviate from factory parts. As I said in post #9, adjust the speed screw for the lowest idle rpm that [a] the engine doesn't stall under braking & the engine sounds/feels comfortable. As an example, you might be able to go down to 400 rpm, but the engine might not sound happy that low, so increase slightly. Change in idle speed should be followed by checking/tweaking the mixture screws.