The carb to tank gasket is there only to prevent leaks. Fuel is pickup via the pickup tube and transferred to the fuel well in the tank. This done with the diaphragm on the side of the carburetor using the vacuum of the intake.
Depending how rusty the tank is as to what might be clogged. In some worst cases I have seen the carburetor tower to have internal rust clogs. But usually the idle jet clogs and the pickup tube. One thing to also check is that the fuel well is in tack and don't have pin holes.
If the tower is clogged you can if you are good remove the steel ball at the bottom and clean out that passage. Just remember to replace this ball with something. Here I use a slightly large larger chrome ball.
And the idle restrictor jet goes in where you marked. Just make sure it is clear of obstructions. It has a pin hole that likes to clog.
Just a side note Briggs has stop making the 3 quart tank last year so they are getting rare to find new. I got one left here as I ordered two before they quit. So hopefully you got the 1 or 2 quart version.
