when selecting a cam, it doesnt matter what engine you are building, I6, V8, heck even a V16 uses the same procedure.
1: determine what rpm range you are planning to run this engine in 80% of the time. if building a street engine, then figure an rpm range of 1000-4500 rpm. then pick a cam that works in that rpm range.
2: decide if you have a lot of low end torque, or not so much, this will help you determine what lobe separation angle you want to run. if your engine doesnt build a lot of low end power, then you want a wide lobe separation angle to trick the engine into thinking it has a higher compression ratio than it really does.
3: decide what rocker arm ratio you are going to run.
once you have decided on these parameters, then you select the cam that closely matches what you are looking for. you can fudge the numbers a bit, but be careful not to go too far one way or another. if there are two cams that meet your parameters, then you can select the cam based on the specs. i value lift over duration for a street cam, the other way around for a race cam.
my mind is aglow with whirling transient nodes of thought careening through a cosmic vapor of invention.