Looks good.
One thing you may want to consider is the collector timing for the scavenging effect.
Ideally, you want the exhaust pulses going around in a circle in order, which is the whole reason for doing the 180° headers. Having the collectors merge like that right where the primaries come together might not allow that to happen. Can you keep a short lenght of secondary tube (10" or so) before bringing them together in the big collector?