Edelbrock 1406 whistling

sounds like you still have your base plate gasket leak.

rather it runs worse sealed up or not......it is suppose to be sealed tight, putting the old gaskets on and allowing it to leak so it runs "better" is not fixing the problem.

sounds like you have more than one problem.

you need to put the new gaskets on and in the correct order so that it is sealed up tight.

at which point it should not be sucking any air in past any gaskets and if it is than it maybe sucking air in at one of the intake gaskets!

have you tried spraying starting fluid along the runners of your intake to see if the idle picks up or bogs down?

do you have your carburetor adjusted?

to adjust the carb let it get up to operating temperature.
start with the screw on the right front of the carb and turn it clockwise until you hear the engines idle start to drop, at which point turn the screw counterclockwise a quarter turn.

repeat these steps on the left side.

I think that is the set the base idle for leaner condition and I think you would do the opposite for a richer tune (can't remember for sure) which you would do the same process but by starting by turning counter clockwise and then a quarter turn clockwise after the change in idle.

this mostly effects just your idle circuit and not your throttle so it likely will not fix any stumble issues BUT it may get rid of the whistle you are hearing.

also you need to check how it runs with choke open and choke cold and both hot and cold operating temps and that may help narrow things down, and of course all the obvious things like fuel filter,how many PSI your fuel pump is pushing,if its electric then do you have your regulator set correctly?,clean air filter,all your vacuum hoses are in good shape and connected?

on the 1406 I believe you have the center front port which is the PCV vacuum,then the left front would be vacuum advance port for your distributor and then you should have another port on the front right that you most likely will have capped off and then you will have the brake booster port on the center rear of the carb. make sure that that booster port aswell as any others are not sucking air somehow .

it definitely sounds like you have a vacuum leak somewhere though being that there is a "whistle" and issues with performance.