Nice, that web site you went to? I'm the lucky guy that maintains it
Those articles came from Jim Hand, the same Jim Hand who wrote this:
And yes, it really is that simple. An engine is nothing more than a big air pump. Make it capable of pumping more air, and you can make more power. All a "big" cam-shaft does is alter the rpm at which the "air pump" is working at its maximum efficiency, getting as much air into the cylinders as possible. Turbochargers and superchargers? All they do is force feed more air into the engine. The more air you can get in, the more fuel you can burn efficiently - and the more fuel you can burn efficiently, the more cylinder pressure you're going to get -- which in an engine translates into more torque. When you use a "big cam" to make the air pump more efficient at a higher rpm, all you're doing is taking advantage of the mathematical formula for horsepower. HP is a calculated value, and that formula is (torque x RPM) / 5250. So, if you have an engine that makes say 500 lb ft of peak torque at 3000 rpm, it's making 285.71 horsepower (at 3000 rpm). Put in a 'bigger' cam such that peak torque of 500 lb ft now occurs at 4500 rpm instead of 3000. Now through the magic of math, the engine is now making 428.57 horsepower! What changed? Peak cylinder pressure is the same, the engine still has a torque peak of 500 lb ft. All we did was move that peak to a higher rpm. Viola. That's all a cam change does - moves the torque peak around by changing the point where the engine is its most efficient.
Go look at any dyno sheet. You will 100% of the time find that the torque and HP curves cross each other at exactly 5250 rpm. That's because of the formula, (torque X rpm) / 5250 = horsepower. When rpm is 5250, it 'cancels out' that 5250 constant in the denominator. At 5250 rpm, every engine's torque and HP numbers will be the same. Always.
Back to your specific question. I'd bet that you'd pick up some torque by getting rid of that 650 Holley and replacing it with a QJet. Why? 650 is borderline too small for a good running Pontiac 400. Even standard model QJets were capable of 750 cfm, and some like the 455-SD version were capable of 800. However, there definitely is such a thing as having a carb that is too big. In order for all of its systems to work properly, a carb depends on a certain minimum air flow velocity going through it. Put on a carb that's "too big" for the engine its on, and the engine won't be able to generate sufficient air flow to make the carb work right. Same with headers - there's such a thing as 'too big' there too. Remember that air (and exhaust gas) has some mass, and therefore has some inertia (Newton's laws of motion). Get your header primary tubes too big, and it's possible for the exhaust gas velocity to get so slow that it will actually stop, and reverse back into the engine the next time the exhaust valve opens. Not only does this take up space in the cylinder that could otherwise be used for fresh air and fuel, now the engine has to expend energy to push this stuff back out. That's energy that won't be going to the rear wheels. It's a balancing act. You want header tubes that will help keep flow velocity up so that when that exhaust valve opens, the spent gas that's already in the header primary tube is still moving outward and will actually help 'suck' things out of the cylinder (this is what is called 'scavenging'), but you don't want tubes SO small that they start causing back pressure - now the engine has to do work again to push stuff out, and we already know we don't want that.
If you really want to get into the details of air flow in an engine, a fellow by the name of David Vizard has some great books out on the subject. Some of them can get pretty 'deep' in places, but I found them fascinating.