The last time mine did that i had knocked a lobe off the cam.You might just have mismatched parts.Did it run okay before,and now you cant get it to tune out?It might not run correctly unless you get better heads put on it,or at least new valve guides and springs and lifters/cam..Now that you have upgraded the intake and carb,the factory heads on your engine might not let the carb and intake have what they need.You may have been blessed with a set of small valve heads,wich dont take to performance upgrades very easily.If the engine is original you might see one of these casting numbers [6259425(350-2bm)],[3970014(350 2 or 4bm)] [3970010(350-2 or 4bm)],as they are the most commonly used engines in your model Chevelle.Others were used as well,but not very often.