Re: Carb won't idle!
As you've deduced, the problem is in the carb somewhere.
If it will only idle at 2000 RPM, the throttle blades are cracked open pretty good, and the idle circuit is out of play.
There's a parts mis-match (someone above said wrong gasket(s), very possible), or serious blockage somewhere, float problem, etc.
Regardless, I'd do a tear-down of the carb and start all over with a new kit designed for your carb (always use a Holley kit).
Rebuilding a Holley only takes about an hour, and that's including the R&R of the carb.