many of the engines i build that is about the first thing i do. is see what they have for a cam core and were i am going for lift and with what rocker ratios. then work out valve springs and spring setup height and coil bind i had a stroker pontiac were i went to a smaller rocker arm as the heads/ valves just would not take the spring setup i needed to run with 1.6 arms would had to really cut the spring pockets that was not going to happen then a custom set longer valve for the taller spring setup was needed