If you are going to venture into header design, do yourself a favor and purchase Pipe Max (
PipeMax36xp2 ). I am unaffiliated, but it is highly regarded at Speed Talk forums, by guys that live and breathe this stuff. From dirt track to Nascar to 4000 hp pro drag cars, there's a lot of big engine builders on there that know there stuff, and trust the software.
Header design is a lot more than target an rpm and pick a diameter/length. Everything from camshaft timing & overlap, to EGT and engine displacement effect what the "best" exhaust is. Collector design is very important as well, be it a straight diameter/length, or getting into merge collectors with taper and volume coming largely into play.
And yes, you don't want to step down to a smaller area than the exhaust port. However, it's not completely unheard of to taper down to a smaller primary to keep velocities up, or form the tube end to fit inside the exhaust port to reduce cross sectional area. A step will cause an unwanted reflection wave back into the port, but a proper taper (<7 deg) will not cause issue. In most cases primaries are larger than the exhaust port, and a step in that direction will not cause so much turbulance, and will help break up the reflected wave from the end of the primary, which can fight low rpm reversion on engines with cam timing favoring higher revs.
The more I try and learn about header and intake design, the more I realize I know nothing. And getting it "right" the 1st time, without a dyno, is a pipe dream. Not saying you can't do something to make improve over stock, at least on older cars. But there's a reason GM will run hundreds of simulations before ever making a part, then test a few dozen different iterations.
Found this one interesting, primary "venturis". Supposably spread the torque curve out, and came in at a lower rpm, with no loss of top end power.
