Be sure to compare the measurements and taper with your old screw. There is more than one possible match for these carburetors. Most screws are 10-32 but the overall length varies.
Measures 10-32 x 1.391 overall length.
Among others fits:
Holley 1909 - You will need 1
Holley 2280 - You will need 2
Ford 4300 - You will need 2
Ford 4350 - You will need 2
Adjusting the Idle Mixture Screws
The idle mixture screws only affects idle. They simply control the amount of fuel and air mixture that is delivered at idle.
First run the screws in all the way until they seat, lightly. Too hard and you will damage the screw. Now back the screw out until the specified rpm is achieved. Turning the idle mixture screws clockwise (in) leans the idle mixture. Turning the idle mixture screws counter clockwise (out) richens the mixture.
Turn the mixture screws, alternating between the two in 1/4 turn at a time until the engine rpm starts to drop, then turn them back out 1/4 of a turn.
When adjusting your idle mixture screws doesn't seem to change the engine rpm much is an indication that there is something wrong and could be an indication of a needed rebuild.