Engine leaning out at 3000rpm

What type of fuel supply system are you running? Also @ 3000 rpm I'm assuming your pretty well putting your foot in it, so if the carb is some type of Holley design does the secondary side have jet extensions on it? After three different carbs on it doing the same thing I'd be leaning towards a fuel supply issue, I'd start by figuring out a way to see what the fuel pressure is doing under load. One other thought is that engine is going to want to eat a lot under power, maybe if you're running a stock gas tank without a lot of gas in it the fuel is sloshing and uncovering the in tank pick up? Without more info on your build these are just some guesses. One other thing that may be a remote possibility, you mention popping out the exhaust, are there any exhaust leaks upstream of where the O2 sensor is located? Air getting into the exhaust from a leak can cause a popping out the exhaust upon deceleration and could be causing the AFR to show lean. More than likely the popping is just do to the mixture leaning out but just a thought.