You want to space the exhaust pulses out in the collector. So on a Mopar with 5-7 that fire next to each other you would put those cylinders into different sides of the collector. If you use a 4-7 swap cam then you might need to change the way a 4-2-1 header is routed.

The Schoenfeld header page shows some of this stuff. They sell 4-2-1 headers for race engines and they'll tell you which ones work with which firing orders.