Optimal intake cross section area and length on normally aspirated engines are mathematically computed and have been known for several decades. Nothing new there. Here's just one of many online calculators.
Intake Runner and Peak Torque Calculator
Generally speaking, you want the plenum volume to be as low as practically possible (for a flat six anyway). If you find that increasing the plenum increases horsepower, then the throttle body is too small. If it's too big throttle response will suffer. Since we're connecting to halves of a flat six, consider each bank of three to have its own plenum, with a big balance tube between.
As the RPM capability of an engine increases, the optimal runner length decreases.
As the engine displacement increases, the intake runner cross section needs to increase.
As far as injector placement, the higher up in the intake runner the injector is, the more power the engine will make. There is no disputing this fact. The only reason that production cars have injectors mounted close to the intake valve is for cold start emissions reduction and idle characteristics, not for performance. Use that info to meet your objectives.
Porsche does a very good job with its intake manifolds and I've found that when checking cylinder head flow rates on a flow bench, the flow will often INCREASE at mid-lift with the intake runner attached. I'd use the Porsche intakes as a guide if I were to build an intake; they've already "done the math."
The latest in intake material tech is plastic. It's very likely that you could 3D print one if you have the capability. Aluminum would be a second choice because of light weight and thermal conductivity.
With turbos, all bets are off. You could pretty much connect everything with a big box and short runners and given enough boost, it'll still make good power. And if it isn't enough, you simply add more boost. Yes, intakes can be optimized with turbo cars, but packaging concerns tend to be the driving factor.