In regards to "proper tuned length", serious dollar race car teams spend lots of money and time with their engines on dynos trying various header lengths and designs to extract every last ounce of horsepower.

The headers are tuned specifically for a particular rev range and induction setup/length.

As every engine is different buying a so called "tuned length" header may be of no extra benefit as it not been tuned for your engine.

Tuned length is a phrase often used but in reality its not even remotely practical for the man on the street, if you had money to burn you could buy various headers try them with various exhausts, dyno them and see what made the most usable power on your car, some guys do that.

A well made header with all the pipes of equal length and an exhaust designed by someone who cares about what they are doing is about the best you can really expect to do.