hi, just wondering if anyone could help me with this.

i have this sat in my garage:









its from my b16a ef crx, as you can see its currently a 4-2-1 degsin.
anyway long story short its cracked on the 2-1 section, so rarther than fix it i want to turn it into a 4-1 header with a 2.5 out pipe/collecter.

could anyone reccomend me what sort of total legnth i need to be using on each primary in total? also for the collecter should would a long nice flowing one be better than a short stubby one ? im lokoing for power from 4000 rpm upwards. engine is stock bar a set of ITR cams/springs installed (its been tuned on BRE from obd0)

thanks