You've asked a question that I'm not sure how to answer
Choosing parts based on targeted outputs. Its a little complicated to describe but simple in theory.
The first and most important part is a realistic idea of what you want. A 700 hp screaming engine is not smart in a 75 Impala Wagon that weighs 5000 lbs, but you obviously already know that. That part of your quest is already established.
For the most part, things will become clear as you read up on things. Comp Cams website is pretty comprehensive as far as tech is concerned. They'll list targeted RPM ranges based on durations. For instance, if you look under their listings for small block chevy, they'll have descriptions of its performance. Their most mild might say something like, "stock converter, 8-9 compresion, good mileage, stock replacement, 800-4000 rpms." The next one might say, "RV and towing, stock converter with 3 series gears, 1000-5000 rpms." The next might say, "mild stall converter, 3.90 gears, 1800-6000 rpms." After that you'll see things like "street/strip," and "race only," and other things.
The biggest choice is determining what is most important to you. For some its mileage, others its cheap gas and therefore compression, for others its being able to operate emissions/vacuum equipment. Then its possible to decide your target HP and your target RPM. For the most part (and this is a Curtis rule, not a standard) is that one HP per CI is always streetable. Another Curtis idea is that with obvious exceptions, most factory assemblies will spin to 6000 rpms without exploding. Those two rules tend to go hand in hand since most carefully chosen combinations will make 1.1 hp per CI or better at about 6000 rpms. You are starting with a 350, so 400 hp (if done well) can be a daily-driver decent mileage engine without sacrificing much at all. So, lets assume you want to make power in the 1000-6000 rpm range. You can now visit your cam's website and choose a cam based on that range. Then visit your favorite intake's site and choose an intake based on that range. When it comes to intakes there are huge grey areas. Your particular 350/400 target suggests either a Performer or Performer RPM intake. For the street I always choose the less radical intake. It will cost you 5 hp, but it makes up for itself in easy cold starts, low end torque, and hood clearance. Now you can visit those head sites and see how much head flow is required to support those HP numbers. Above all, I suggest buying Desktop Dyno or a similar dyno software program. Read the manual and directions from start to finish since they offer invaluable tips for making the software accurate. I used Desktop Dyno to help my recomendations for you.
Its important to recognize that these dyno programs are not entirely accurate as far as predicting nominal output, but they use volumetric theory to determine efficiency. They disregard things like friction and assume things like a proper fuel mix, but they will show you a pretty accurate estimate of your output.
I start with a target output, which is partly from experience. I know that I could build a 900-hp 350, but it will cost me $10,000 and won't make power until 6000 rpms. That's why I stick to my standard rule of 1.1 hp per CI for the mild street engine on basic over-the-counter technology. You'll learn as you go along from experience, but start with a target. If your target is too lofty you'll realize it as you go through the process. You'll get to a point where you see that in order to support that kind of horsepower its going to require $1600 heads and a cam that makes no power below 3000.
There are certain things to remember as you go through this. You know that Quality of airflow is as important as Quantity. It stands to reason therefore that a small block Chevy 350 has more streetable power potential than, say an International Harvester 345. The aftermarket has poured billions into chevy design and they've pushed the limits of the big flow/small port possibilities. The IH engine had three head castings all of which sucked. In order to get them to flow the same as a well designed Edelbrock chevy head, they would have to be massively ported and therefore make the ports big and velocity slow. It is therefore harder to make 1.1 hp/ CI in the IH engine without killing low end torque and therefore streetability.
Once you've visited all of your websites adn determined your targets you can plug the parameters into your dyno simulation software. Sometimes you are pleasantly surprised. Other times you may realize you are below where you want to be. Then you can go back and adjust things. Choose a larger cam and intake and see what it does to your curves. The important thing is that all the components must match or it will hurt drivability and power. The good news is that components like cams and intakes are published with their peak power ranges. Matching them is easy.